feat: forward proposedSessionId in session/new via createSession hints

[henkterharmsel]

What

Two related fixes for reliable ACP session resume in `acpx`:

1. `createSession(cwd, hints?)` — forward `proposedSessionId`

`createSession` now accepts an optional `hints` object. When `hints.proposedSessionId` is provided, it is included in the `_meta` field of the `session/new` ACP request.

All internal `createSession` call sites in the runtime engine (`manager.ts` and `reconnect.ts`) now derive the session UUID from the ACP session key (`agent:claude:acp:` → `.split(':').pop()`) and pass it as `proposedSessionId`. This gives Claude Code sessions a stable, predictable file name — required for reliable resume.

2. Oneshot resume double-load fix

`ensureSession` (phase 1) previously started Claude, loaded the session, then shut it down via `client.close()`. This wrote a `closed` marker to the JSONL file. `connectAndLoadSession` (phase 2) then failed because the JSONL was already closed.

For oneshot sessions with a `resumeSessionId`, `ensureSession` now skips the start/load/close cycle entirely, constructs a minimal session record, and returns early. Phase 2 performs the actual resume.

Why

Without these fixes, resuming a completed oneshot session in OpenClaw always returns "Internal error". New sessions work fine; it is specifically resume that fails.

Root causes:

1. `claude-agent-acp` used `randomUUID()` for every session, making file names unpredictable.
2. `acpx` did not support passing `proposedSessionId` through `createSession` or its internal call sites.
3. `ensureSession`'s two-phase design closed the JSONL before phase 2 could use it.

Backward Compatibility

Both changes are additive. Existing behavior is preserved for non-oneshot modes and sessions without a `resumeSessionId`. Passing `proposedSessionId` is opt-in.

Dependencies

This PR is part of a two-repo fix for ACP session resume. Merge order:

1. `fix: deterministic session IDs and reliable resume for ACP sessions agentclientprotocol/claude-agent-acp#554` — support `proposedSessionId` in `_meta` and fix UUID extraction (merge first)
2. `openclaw/acpx` — this PR (merge after Align CI/test setup with SimpleDoc and expand coverage #1)

Note: an openclaw PR is not required. OpenClaw already passes the correct `resumeSessionId` and `sessionKey` parameters to `ensureSession` — all fixes are encapsulated in `acpx`.

Each PR is backward-compatible with the previous version of its dependency.

Testing

• New tests added for `proposedSessionId` forwarding (in `createSession` and reconnect fallback paths), oneshot resume bypass, and UUID extraction.
• Full test suite passes: `pnpm test` (527/527)
• Build and lint clean: `pnpm build && pnpm check`
• End-to-end verified locally: OpenClaw oneshot sessions resume correctly with multi-turn history preserved (OpenClaw 2026.4.12 + acpx 0.5.3 + claude-agent-acp 0.27.0).

AI Assistance 🤖

Developed with AI assistance (Claude Code). Logic derived from locally-applied patches confirmed working. Author (Henk ter Harmsel) has reviewed and understands all changes.