feat(acp): complete capability matrix

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Summary

Complete the Crabcode ACP capability matrix and make crabcode acp fully functional across session lifecycle, prompts, tools, permissions, commands, MCP, terminals, questions, cancellation, and usage accounting.

Highlights

• complete persisted session lifecycle with cursor pagination, hierarchy metadata, stable message IDs, fork isolation, close, and delete
• support persistent image/audio prompt attachments with legacy attachment migration and WAV/MP3 audio routing
• stream full and structured tool results, normalized locations, native diffs/media/resources, and MCP annotations/metadata
• preserve real permission tool-call IDs and provide non-mutating preflight diffs, including multi-file apply_patch
• forward interactive questions through capability-gated ACP form elicitation
• embed client-hosted terminals with create/wait/output/kill/release and cancellation support
• add ACP /compact, local /skills and /mcp, custom command overrides, and explicit unknown-command errors
• retain provider stop reasons and authoritative input/output/cache usage with persisted, cache-aware cost accounting
• add stdio subprocess initialization and clean-EOF integration coverage
• update _docs/acp.mdx so all Crabcode-side capability areas are marked Full, with editor/model/protocol requirements documented separately

Storage and compatibility

• adds persistence fields/migrations for authoritative usage and durable attachment metadata
• attachments are stored per session under Crabcode state storage and copied independently on fork
• _meta.crabcode extensions expose hierarchy and detailed usage while preserving ACP compatibility
• unsupported future MCP content remains available through lossless raw/text fallback

Validation

• focused ACP, session, persistence, provider, MCP, permission, question, terminal, attachment, audio, compaction, and tool tests pass
• ACP stdio subprocess lifecycle test passes
• cargo check passes
• cargo fmt passes
• scripts/check-aisdk-boundary.sh passes

The repository-wide suite still exposes pre-existing shared global test-state coupling when session tests run in one process. Those session tests pass with isolated XDG_STATE_HOME; two unrelated existing failures remain in model-cache fixture and renderer-path assertions.