refactor(agent): migrate ACP adapter to agent-client-protocol 1.2

[Tryanks]

Supersedes #63. The 0.10 → 1.2 jump crosses ACP's protocol-v2/schema redesign; this is a structural migration of the ~2800-line adapter, not a rename pass.

Key decisions:

• Stays on the stable schema::v1 wire surface (ACP's stable protocol is still v1; v2 requires an explicit unstable feature). Dropped the broad unstable feature; retained only unstable_auth_methods and unstable_end_turn_token_usage.
• ClientSideConnection/async Client trait → 1.2 Client.builder() + typed handlers + ByteStreams; dedicated LocalExecutor thread preserved; handler state moved to Arc<Mutex<_>> for the new Send callback requirements; permissions/fs/terminal handlers spawned off the dispatch loop.
• Model selection migrated to categorized session config options (session/set_config_option).
• Semantics preserved: JSONL-authoritative resume (ACP replay swallowed; failed load → new session + warning), approval mapping, interruption, stop-reason/usage, MCP capability gating, path validation, terminal output.
• All 25 ACP tests retained (one SessionModelState fixture adapted to schema 1.4's config-option equivalent, assertions kept + one added). Known edge: legacy agents exposing only the removed unstable models extension (without categorized config) lose model selection — no stable RPC exists for that in 1.2.

Gates: fmt / clippy -D warnings / workspace tests (agent: 116 passed incl. full ACP suite) green locally.

🤖 Generated with Claude Code