chore: add Agent Plugin package for the a2a-cli skill

agent-plugin/LICENSE

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agent-plugin/README.md

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+# a2a-cli — Agent Plugin
+
+An [Agent Plugin](https://agent-plugins.org/) that teaches a plugin-aware AI agent how to drive the [`a2a` CLI](https://github.com/a2aproject/a2a-cli) — discovering, messaging, and managing A2A (Agent2Agent) agents from the command line.
+
+It packages the a2a-cli Agent Skill as one installable unit for a plugin-aware client. The skill and the binary can always be installed independently — the plugin is a convenience, never a precondition.
+
+## Contents
+
+```text
+agent-plugin/
+├── plugin.json                 # manifest (Agent Plugins 1.0.0)
+├── LICENSE                     # Apache-2.0
+└── skills/
+    └── a2a-cli/
+        └── SKILL.md            # agent-facing usage guidance
+```
+
+Skills-only: this plugin carries no MCP server, which is valid under Agent Plugins (a skills-only plugin conforms).
+
+## Requires the `a2a` binary
+
+Agent Plugins packages the agent-facing pieces (skills and MCP servers), not tool binaries. The `a2a` binary is **not** bundled and must be installed separately (Homebrew, WinGet, a prebuilt release, or `go install`) — see the [project README](https://github.com/a2aproject/a2a-cli#installation) and the skill's own Setup section. The skill includes a preflight check (`a2a version`).
+
+## Install
+
+With the [`skills`](https://github.com/vercel-labs/skills) CLI, from this repository:
+
+```bash
+# from a local checkout
+npx skills add ./agent-plugin/skills/a2a-cli -g
+
+# or directly from GitHub
+npx skills add https://github.com/a2aproject/a2a-cli/tree/main/agent-plugin/skills/a2a-cli -g
+```
+
+A plugin-aware client can instead load this directory as an Agent Plugin: it reads `plugin.json`, discovers skills under `skills/`, and validates each `SKILL.md`. Installation, enablement, and updates are client-controlled.
+
+## Versioning
+
+The plugin `version` in `plugin.json` (`0.2.0`) tracks the `a2a` CLI release; the bundled skill keeps its own date-based `metadata.version`. The manifest targets Agent Plugins spec **1.0.0**.

agent-plugin/plugin.json

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+{
+  "$schema": "https://agent-plugins.org/schemas/1.0.0/plugin.schema.json",
+  "name": "a2a-cli",
+  "version": "0.2.0",
+  "description": "Drive A2A (Agent2Agent) agents from the command line with the a2a CLI: fetch an agent card, send a message, stream or poll a task, and list, resume, or cancel tasks.",
+  "author": {
+    "name": "A2A Project",
+    "url": "https://a2a-protocol.org"
+  },
+  "homepage": "https://github.com/a2aproject/a2a-cli",
+  "repository": "https://github.com/a2aproject/a2a-cli",
+  "license": "Apache-2.0",
+  "keywords": ["a2a", "agent2agent", "cli", "agents", "interoperability", "agent-card"]
+}

agent-plugin/skills/a2a-cli/SKILL.md

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+---
+name: a2a-cli
+description: >-
+  Delegate work to A2A (Agent2Agent) agents from the command line with the `a2a`
+  CLI (github.com/a2aproject/a2a-cli). Use when you need to hand a task to a
+  specialized or remote agent, fetch an agent's card, send a message to an agent
+  at a URL, stream or poll a running task, or check, list, resume, or cancel an
+  A2A task. Not for building or serving an A2A agent, or for calling non-A2A
+  HTTP APIs.
+compatibility: >-
+  Requires the `a2a` binary on PATH. Install it with Homebrew or WinGet, from a
+  prebuilt release binary, or from source with a Go toolchain. The skill installs
+  nothing itself.
+license: Apache-2.0
+metadata:
+  source: https://github.com/a2aproject/a2a-cli
+  version: "2026.09.08"
+---
+
+# Driving A2A agents with the `a2a` CLI
+
+`a2a` is a stateless client for the [A2A protocol](https://a2a-protocol.org/):
+give it an agent and a message, it negotiates the transport from the agent's
+card (JSON-RPC, REST, or gRPC), sends the message, and reports what the agent
+returned.
+
+## Core concepts
+
+- An interaction starts with a **Message** you send to an agent.
+- The agent replies with either a **Message** (a direct answer) or a **Task**
+  (tracked, potentially long-running work).
+- A **Task** has a server-assigned `taskId` and a `contextId`, and moves through
+  states. It can complete, fail, be cancelled, or become **interrupted** when it
+  needs your input or authentication to continue.
+- Resume an interrupted task by sending a Message with its `--task-id`.
+- Group related work with `--context-id`: a Task created in reply joins that
+  context. Many tasks can share one context, and a `--task-id` and `--context-id`
+  passed together must agree.
+- The server keeps tasks (and the messages tied to them); a plain Message not
+  tied to a task is not stored — so **you** hold the `taskId`/`contextId` to
+  return to work later.
+
+## Setup
+
+The skill needs the `a2a` binary on PATH — check with `a2a version`. If it is
+missing, install it with Homebrew (`brew tap a2aproject/a2a-cli
+https://github.com/a2aproject/a2a-cli && brew install a2a`), WinGet
+(`winget install a2aproject.a2acli`), a prebuilt binary from the
+[releases page](https://github.com/a2aproject/a2a-cli/releases/latest), or from
+source with a Go toolchain (re-run to update):
+
+```bash
+go install github.com/a2aproject/a2a-cli@latest
+
+# rename to a2a to match the docs
+
+mv "$(command -v a2a-cli)" "$(dirname "$(command -v a2a-cli)")/a2a"
+```
+
+The tool is under active development. Treat `a2a help` and `a2a <command>
+--help` as the source of truth for the current commands and flags.
+
+## Task lifecycle
+
+Point at an agent with `-a <host|url|path>` (resolves its card and picks a
+transport) or `-e <url> --transport <rest|jsonrpc|grpc>` (connect directly).
+
+1. **Inspect the agent** — confirm it is reachable and see what it supports:
+
+   ```bash
+   a2a card get https://agent.example.com
+   ```
+
+2. **Send a message.** `send` blocks until the task reaches a terminal or
+   interrupted state. Add `-o json` for machine-readable output, and note the
+   `taskId` and `contextId` in the response — you need them to continue:
+
+   ```bash
+   a2a send -a https://agent.example.com "Summarize this repo"
+   a2a send -a https://agent.example.com -o json "Summarize this repo"
+   ```
+
+3. **Follow a long task live** instead of blocking, or re-attach to one later:
+
+   ```bash
+   a2a send -a https://agent.example.com --stream "Run a long analysis"
+   a2a task subscribe -a https://agent.example.com <task-id>
+   ```
+
+4. **Check status and fetch results** at any time:
+
+   ```bash
+   a2a task get -a https://agent.example.com <task-id>
+   ```
+
+5. **Answer an interrupted task** (it reached `INPUT_REQUIRED` / `AUTH_REQUIRED`)
+   by replying on the same task:
+
+   ```bash
+   a2a send -a https://agent.example.com --task-id <task-id> "Yes, proceed"
+   ```
+
+6. **Continue the conversation** as a new task in the same context:
+
+   ```bash
+   a2a send -a https://agent.example.com --context-id <context-id> "Follow-up question"
+   ```
+
+7. **List or cancel tasks:**
+
+   ```bash
+   a2a task list -a https://agent.example.com
+   a2a task cancel -a https://agent.example.com <task-id>
+   ```
+
+## Key flags
+
+Run `a2a <command> --help` for the full, current set. The load-bearing ones:
+
+| Flag | Use |
+|---|---|
+| `-a, --agent-card <host\|url\|path>` | Resolve the agent's card (picks the transport). |
+| `-e, --endpoint <url>` + `--transport <rest\|jsonrpc\|grpc>` | Connect to one interface directly, skipping card resolution. |
+| `-o, --output json` | Machine-readable output; add `--stream` for a live event stream. |
+| `--async` | Return immediately with the identifiers instead of blocking; poll later with `task get`. |
+| `--task-id <id>` / `--context-id <id>` | Continue a task / group a new task under a context. |
+| `--auth "<creds>"` / `--svc-param <k=v>` | Attach credentials or transport parameters (or set `A2ACLI_*` env vars). Never commit a secret. |
+
+## Configuration
+
+Every setting can come from a flag, an `A2ACLI_*` environment variable, or a
+`.env` file (a local `.env`, or `~/.config/a2a-cli/.env`); precedence is
+flag > env var > file. Inspect the effective values and where each resolved from
+with `a2a config show` (secrets redacted).