Enable manual compaction for built-in Grok ACP provider

[bb-slop-cop]

Human comments

What was wrong

The built-in Grok ACP provider inherited the ACP default of supportsManualCompaction: false. The server rejected compaction with HTTP 409 before dispatching to the ACP bridge. The verified reproduction and root-cause report records the failure in two clean checkouts.

What changed

Enabled manual compaction for the built-in Grok ACP provider, extended the public thread compaction test to cover its request, updated the existing provider expectation, and documented the CLI behavior in the ACP provider skill. No protocol or wire fields changed. The diff has 76 changed text lines across four files, with no binary changes.

How you verified

• The focused Grok route regression failed before the fix: HTTP 409 where HTTP 200 and a structured compaction turn were expected.
• After the fix, the public compaction and first-party provider suites passed: 8 tests.
• ACP provider agent suites passed: 28 tests.
• pnpm exec turbo run lint typecheck --filter=@bb/server --filter=bb-plugin-provider-acp passed.
• git diff --check origin/main passed.

Fixes #4378

AGENT GENERATED