feat(acp): IDE-proxied file system, terminal, permissions, MCP passthrough

[bug-ops]

Summary

Phase 2 of ACP integration — IDE-proxied capabilities for zeph-acp crate:

• acp: IDE-proxied file system operations #767 fs.rs — AcpFileExecutor implementing ToolExecutor for read_file/write_file proxied through IDE via ACP Client trait, with absolute path validation and traversal rejection
• acp: IDE-proxied terminal execution #768 terminal.rs — AcpShellExecutor implementing ToolExecutor for bash commands proxied through IDE terminal lifecycle (create/wait/output/release), with permission gate integration
• acp: permission request flow for tool execution #769 permission.rs — AcpPermissionGate with session-scoped tool_name cache for AllowAlways/RejectAlways decisions, mpsc+oneshot bridge pattern for !Send conn access
• acp: MCP server passthrough from IDE client #770 mcp_bridge.rs — convert IDE-provided MCP server configs (stdio transport) to zeph-mcp ServerEntry format
• agent.rs — AcpContext struct, updated AgentSpawner signature to accept Option<AcpContext>
• error.rs — ClientError, CapabilityUnavailable, ChannelClosed variants

Architecture

All adapters use channel bridge pattern (mpsc sender in Send context, LocalSet receiver calls AgentSideConnection methods, oneshot reply):

Agent thread (Send)          LocalSet (!Send)
       |                          |
  AcpFileExecutor  --[mpsc]--> fs_handler(conn)
  AcpShellExecutor --[mpsc]--> terminal_handler(conn)
  AcpPermissionGate--[mpsc]--> permission_handler(conn)
       |                          |
  <--[oneshot]-- reply            |

Known limitation

transport.rs conn-sharing wiring (spawning LocalSet handlers with Rc<AgentSideConnection>) is deferred — adapters are implemented and tested but not yet connected to the real connection. This will be completed in Phase 3 alongside integration tests (#773).

Closes #767, #768, #769, #770

Test plan

• 40 unit tests pass (cargo nextest run -p zeph-acp)
• 2554 workspace tests pass
• Security audit: path traversal validation, permission gate on shell, session-scoped cache
• Performance review: no blockers, channel overhead acceptable
• cargo clippy --workspace -- -D warnings clean
• cargo +nightly fmt --check clean