feat(acp): add Agent Client Protocol support with daemon relay

[Wirasm]

Summary

• Problem: KILD communicates with AI agents via raw PTY byte streams, limiting structured interaction (no typed tool calls, permission flows, or plan visualization)
• Why it matters: ACP (JSON-RPC 2.0 over stdio) is an industry standard adopted by 25+ agents — integrating it transforms KILD from a terminal multiplexer into a structured agent orchestration platform
• What changed: New kild-acp crate, RuntimeMode::Acp variant, ACP daemon relay (transparent byte tunnel), --acp CLI flag, agent backend ACP command declarations, UI awareness
• What did not change (scope boundary): No rich ACP panel view (Phase 2), no ACP message parsing in daemon (intentionally transparent relay), no changes to existing PTY session flow

Label Snapshot (required)

• Risk label: risk: medium
• Size label: size: L
• Scope labels: core, agent, daemon, runtime
• Module labels: agent:claude, agent:gemini, agent:opencode
• Contributor tier label: N/A
• If any auto-label is incorrect, note requested correction: N/A

Change Metadata

• Change type: feature
• Primary scope: runtime

Linked Issue

• Closes #
• Related #
• Depends on #
• Supersedes #

Validation Evidence (required)

cargo fmt --check              # ✓ clean
cargo clippy --all -- -D warnings  # ✓ clean
cargo test --all               # ✓ 2,493 tests pass, 0 failures
cargo build --all              # ✓ clean

• Evidence provided: Full build + test + lint pass. E2E manual test: kild create test-acp --acp → daemon spawned claude-code-acp (PID confirmed) → kild list showed runtime_mode: acp → kild stop killed process → kild destroy cleaned up.
• If any command is intentionally skipped, explain why: N/A

Security Impact (required)

• New permissions/capabilities? No
• New external network calls? No
• Secrets/tokens handling changed? No
• File system access scope changed? No

Privacy and Data Hygiene (required)

• Data-hygiene status: pass
• Redaction/anonymization notes: N/A
• Neutral wording confirmation: Yes

Compatibility / Migration

• Backward compatible? Yes — ACP is opt-in via --acp flag, all existing PTY sessions unchanged
• Config/env changes? No
• Migration needed? No

Human Verification (required)

• Verified scenarios: Full E2E flow — create ACP session with claude-code-acp, verify process running via ps, check runtime_mode: acp in JSON list output, stop session, destroy session
• Edge cases checked: --acp conflicts with --daemon/--no-daemon (clap error), attach refused for ACP sessions
• What was not verified: ACP message-level protocol handshake through kild-acp crate (requires Phase 2 UI integration), opencode/gemini ACP agents

Side Effects / Blast Radius (required)

• Affected subsystems/workflows: New kild-acp crate (standalone, no internal deps), kild-protocol (new message types — non_exhaustive enums), kild-daemon (new acp/ module), agent backends (new trait method with default None), CLI (new flag), UI (badge + guard)
• Potential unintended effects: None — ACP is entirely additive, existing PTY path untouched
• Guardrails/monitoring for early detection: All existing tests pass unchanged, new tests cover all new code paths

Rollback Plan (required)

• Fast rollback command/path: git revert <commit> — single commit, clean revert
• Feature flags or config toggles: --acp flag is opt-in, no config changes
• Observable failure symptoms: kild create --acp would fail to spawn agent process, kild list would not show ACP sessions

Risks and Mitigations

• Risk: ACP SDK (agent-client-protocol v0.9) is pre-1.0 and API may change
  • Mitigation: kild-acp crate is a standalone wrapper with no internal deps — SDK upgrades are isolated to one crate
• Risk: Daemon relay doesn't validate ACP message framing (transparent byte tunnel)
  • Mitigation: Intentional design — protocol parsing happens in client (kild-acp), daemon stays simple. Malformed messages fail at the client layer.

Architecture

Client (kild-ui/CLI)                 kild-daemon                    Agent
  CreateAcpSession ────────────────→  spawn agent --acp
  AcpRelay { data } ──────────────→  write to stdin ──────────────→ JSON-RPC
  ← AcpRelay { data } ←────────────  read from stdout ←─────────── JSON-RPC
  ← AcpSessionExited ←─────────────  detect process exit

New crate: kild-acp

Wraps the agent-client-protocol SDK behind a Send+Sync API. The SDK produces !Send futures, so the crate spawns a dedicated OS thread with a single-threaded tokio runtime + LocalSet. External callers communicate via mpsc channels.

Files changed (32 files, +1698/-71)

File list

• Cargo.lock / Cargo.toml — workspace dependency additions
• crates/kild-acp/ — new crate (6 files: lib, errors, types, client_impl, connection, runner)
• crates/kild-protocol/src/types.rs — RuntimeMode::Acp
• crates/kild-protocol/src/messages.rs — 6 new ACP message types
• crates/kild-core/src/agents/traits.rs — acp_command() method + AcpCommandInfo
• crates/kild-core/src/agents/backends/mod.rs — 2 new macro arms for ACP agents
• crates/kild-core/src/agents/registry.rs — get_acp_command(), supports_acp()
• crates/kild-core/src/agents/mod.rs — re-exports
• crates/kild-core/src/daemon/client.rs — create_acp_session()
• crates/kild-core/src/sessions/daemon_spawn.rs — spawn_acp_agent()
• crates/kild-core/src/sessions/daemon_helpers.rs — env var builder for ACP
• crates/kild-core/src/sessions/create.rs — ACP runtime mode dispatch
• crates/kild-core/src/sessions/open.rs — ACP session reopen
• crates/kild-daemon/src/acp/ — new module (mod.rs + relay.rs)
• crates/kild-daemon/src/server/connection.rs — dispatch 3 ACP message types
• crates/kild-daemon/src/session/manager.rs — create_acp_session(), write_acp_stdin()
• crates/kild-daemon/src/lib.rs — acp module declaration
• crates/kild-ui/src/views/sidebar.rs — ACP badge
• crates/kild-ui/src/views/main_view/kild_handlers.rs — terminal tab guard
• crates/kild/src/app/session.rs — --acp flag definition
• crates/kild/src/commands/create.rs — read acp flag
• crates/kild/src/commands/open.rs — [acp] mode label
• crates/kild/src/commands/attach.rs — refuse attach for ACP
• crates/kild/src/commands/helpers.rs — resolve_runtime_mode() with ACP