feat(agent-history): add Antigravity CLI (agy) history reader

[dorokuma]

What

Adds an AgentHistoryReader for Antigravity CLI (agy), so Shepherd can surface the last assistant message for agy agents managed through Herdr (agent_session refs with source herdr:antigravity_cli previously resolved to unknown, leaving wake summaries empty).

How

• New AgyHistoryReader parses ${HOME}/.gemini/antigravity-cli/brain//.system_generated/logs/transcript.jsonl. Line mapping: USER_INPUT -> user; PLANNER_RESPONSE with text content -> assistant; PLANNER_RESPONSE with only tool_calls -> tool_result (tool name from the first call). Malformed lines are skipped, matching the existing readers.
• historySourceFromSessionRef maps agent agy / antigravity* refs to the new agy-jsonl source. Discovery resolves kind: id refs to the transcript path and scans the brain directory (most recently modified transcript first).
• Directory auto-discovery filters candidate transcripts by the agent's cwd using conversation_summaries.db (workspace_uris), so concurrently running agy agents do not pick up each other's transcripts. When no conversation matches the cwd, discovery returns no history instead of an unrelated one. If the summaries database is missing or unreadable, discovery falls back to the newest transcript.
• The agy-jsonl source type is appended to the AgentHistoryRef union; AgyHistoryReader is appended to the default reader list. No existing reader is touched.

Testing

• 12 new unit tests: multi-turn parsing, tool-only responses, empty file, malformed JSON lines, missing file, session-id path resolution, default service registration, source mapping, brain directory discovery, mtime ordering across multiple sessions, cwd filtering via conversation summaries, no-cwd-match returning no history.
• Full suite: 239 tests passing. tsc --noEmit, biome check, and build are clean.

Note: agy transcripts do not embed a cwd; directory auto-discovery matches the agent's cwd against conversation_summaries.db when available and falls back to the newest transcript otherwise.

[ryonakae]

Thanks for working on Antigravity history support. I verified that the agy agent name and herdr:antigravity_cli session source match Herdr's integration, and this would be a useful addition to Shepherd.

I found a few correctness issues that should be addressed before merging:

1. PLANNER_RESPONSE.tool_calls[].args is currently emitted as a tool_result. These are tool inputs, not execution results. Actual results appear in separate transcript entries such as LIST_DIRECTORY. This can make agent read, lastToolResult, and wake notifications report incorrect information. Please parse actual result entries, or omit tool results when they cannot be represented correctly. Tests should cover actual output, errors, and multiple tool calls.

2. When conversation_summaries.db is missing or unreadable, discovery falls back to the newest transcript across the entire brain directory. Since these transcripts do not contain a cwd, this can attribute a conversation from another workspace to the current agent. Please fail closed in this case: returning no history is preferable to returning another agent's history.

3. Workspace URI matching currently accepts parent/child relationships in both directions and treats / as matching everything. This can mix root and subproject conversations. Please use proper file URI/path normalization and restrict matching to an unambiguous relationship.

Please also update the supported-agent list in README.md and README.ja.md.

The overall feature is worth merging; these are correctness issues rather than a rejection of the approach. I'll be happy to review an updated version.