Add agent-scoped skill configuration

[artjen]

Why

Developers need to expose a skill schema to one coding agent without also exposing it to every other configured agent. This prevents unintended skill-tool crossover between agents.

What changed

This PR adds --agents to ucode skill add --mcp and documents agent-scoped additions.

How it works

• Named agents are set up before their MCP configuration is changed.
• Without --agents, locations are added to the shared default.
• With --agents, locations are added only to each selected agent's durable additions.
• Adding a location already present in the default still records the explicit agent intent.
• Download mode rejects --agents because its on-disk skill directories are shared.

Builds on the client-aware state model in #468.

Testing

• uv run pytest tests/test_skills_download.py tests/test_mcp.py tests/test_cli.py -q — 501 passed
• uv run ruff check .
• uv run ty check src/

Tests cover global and targeted persistence, unaffected client URLs, durable overlapping additions, CLI forwarding, and download-mode validation.

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[Copilot]

🟢 Approval recommended

The implementation matches the stated behavior, is well-covered by targeted tests, and preserves backward-compatible defaults for existing users.

Pull request overview

This PR extends ucode skill add --mcp to support agent-scoped skill schema additions via a new --agents option, enabling one agent’s skills MCP scope to be updated without affecting other configured agents.

Changes:

• Add --agents to ucode skill add --mcp, wiring it through CLI parsing/setup into MCP state updates.
• Update skills MCP state so agent-scoped additions persist under per-client overrides while leaving the shared default scope intact.
• Add unit tests and README documentation covering agent-scoped behavior and download-mode validation.

File summaries

File	Description
src/ucode/mcp.py	Extend add_skills_command to optionally write per-agent skill scope overrides and update only selected clients.
src/ucode/cli.py	Add --agents to skill add, validate it is MCP-only, and forward the resolved agent scope to add_skills_command.
tests/test_mcp.py	Add coverage for agent-scoped additions, preserving overrides during global additions, and persisting explicit intent.
tests/test_cli.py	Verify CLI agent scope setup/forwarding for --mcp and rejection of --agents in download mode.
README.md	Document agent-scoped MCP additions and clarify why download mode rejects --agents.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0
• Review effort level: Lite

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[Copilot]

🟢 Approval recommended

The implementation matches the described behavior, preserves existing scope semantics, and includes focused tests covering agent-scoped persistence and CLI validation paths.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0 new
• Review effort level: Lite