chore: promote reasoning distillation for v1.0.51

[LeXwDeX]

Why

Promote the completed opt-in reasoning distillation implementation to the stable release branch for GraphAgent v1.0.51.

What changed

Promotes #667 and the implementation tracked by #666. opencode.json defaults reasoning distillation to off; enabling it synchronously adopts the first completed user turn and asynchronously adopts each later turn. Accepted reasoning replaces durable history and live TUI content, survives reopening, and is reused by subsequent model requests. Failed, cancelled, disabled or stale work retains current text.

Includes the exact v1.0.51 release notes. There are no unrelated source changes relative to the previous stable release.

Evidence

• Complete local workspace tests: 8/8 tasks successful; host 4881 passed with 0 failures; Core 1441 passed. Existing 30 skipped host tests and one todo remain unchanged.
• Typechecks, lint, DAG gate, generated clients, installer checks, Go configuration tests and the three HTTP API modes passed locally.
• The compiled macOS binary passed isolated model-fixture tests for missing/false configuration, first synchronous adoption, later asynchronous adoption and downstream replay.
• A live pseudo-terminal displayed actual model-driven adoption and retained it after reopening. Local fixtures validate application and transport behavior, not production upstream semantic quality.
• Native CI acceptance is required on this promotion before merge. Main release assets and their checksums/runtime behavior will be verified separately after publication.

Final stable acceptance: graphagent-v1.0.51 is published, non-draft, non-prerelease and Latest. Tag, template runtime provenance and accepted main all resolve to 6bb00086382653d0c90b32174d98ff7ddd27f821. Release run 36379736223 succeeded after the notes-format correction in #670. Current main Typecheck, Linux unit/API, Linux/Windows E2E and CodeQL all succeeded. The downloaded macOS archive matches SHA256SUMS and is byte-identical to the tested final candidate: 4d6cbd3571829cd5d8035a75da53a5cb7460c68fee219a967e69b96c224ce3a5. It passed 6 installer/signature checks plus isolated compiled first-turn sync, subsequent async, live TUI replacement, reload, downstream replay and missing/false opt-out tests. The local model fixture verifies lifecycle/transport/display behavior, not production-model semantic fidelity.

Checklist

• Implementation and local validation completed.
• Stable release notes included.
• Current-head Typecheck, Linux unit tests and Linux/Windows E2E accepted.
• Main stable release published and downloaded artifact verified.