feat(ide): run ACP sessions on the full thread/turn runtime; fix stale RUNTIME_API.md ACP docs (IDE stage 2)

[Hmbown]

Part of the ratified IDE program (founder 2026-09-02; PRD §6 "IDE direction"). Stage 2: Codewhale as an agent option in ACP hosts (Zed, JetBrains, VS Code, Neovim all host ACP agents today).

Current state

crates/tui/src/acp_server.rs (codewhale serve --acp) implements initialize / session/new / session/prompt / session/cancel, streams session/update chunks, executes tools locally through a ToolRegistry built from the same file/search/git/patch/shell tools as the CLI exec agent, and gates them via session/request_permission.

Two gaps:

1. Session weight: the header documents that prompts route through the same configured client as one-shot CLI mode with a per-session tool loop — not the full thread/turn runtime. ACP sessions therefore lack durable threads, snapshots, steering, and approval parity with the Runtime API.
2. Stale docs: docs/RUNTIME_API.md (~lines 293-310) claims ACP "does not yet expose shell tools, file-write tools" — false against the current code.

Deliver

• ACP sessions backed by the same thread/turn engine as /v1/* (durable threads, resume, snapshots where the ACP surface allows), with session/request_permission mapped onto the engine's approval model (AskForApproval, execpolicy).
• Rewrite the ACP section of docs/RUNTIME_API.md to match reality.
• A conformance smoke test against a real ACP host (Zed is the reference implementation of the client side).

Acceptance

• A Zed-hosted Codewhale session survives editor restart (thread resume).
• Permission prompts in the host map to engine approvals with the same policy semantics as the TUI.
• Docs match the code; stale claim removed.