Wire AcpContext through AgentSpawner into agent tool chain

[bug-ops]

Parent Epic

#777

Problem

In main.rs, _acp_ctx is discarded in the spawner closure. AcpFileExecutor, AcpShellExecutor, and AcpPermissionGate are constructed but never used.

Solution

• Accept AcpContext in spawn_acp_agent() and build_acp_deps()
• When AcpContext.file_executor is Some, use it instead of local file tools
• When AcpContext.shell_executor is Some, use it instead of ShellExecutor
• When AcpContext.permission_gate is Some, wrap tool execution with permission checks
• Integrate via CompositeExecutor or a new AcpToolExecutor wrapper

Critical Path

No dependencies — can start immediately.
Blocks: #780 (cancellation), Epic 2 (#778)

Acceptance Criteria

• AcpContext parameter used (not discarded) in spawner
• IDE FS read/write proxied when capability advertised
• IDE shell execution proxied when capability advertised
• Permission gate wraps tool calls when capability advertised
• Fallback to local tools when IDE capability not advertised
• Tests with mock ACP client verifying tool delegation