feat(coding-agent): refine.preview and pinned refine.run plan_id

[nate-benhra]

Adds a preview/approve flow for continual harness refinement (closes #684).

Commit 1 — refine.preview(): new kernel host request that runs the existing planning phase (_planRefine) and returns the proposed edits (plan_id, summary, rationale, expected outcome, per-edit action/kind/id/title/content/reason) without scheduling or applying anything. refine.status() now also reports the pending request content and cached preview ids. Preview waits for planning quiescence before claiming the refine abort-controller slot, so it never races a serialized background plan.

Commit 2 — refine.run(plan_id=...): applies exactly the previewed plan instead of re-planning. Previewed plans are cached on the session (cap 5) with branch-version validity, and are invalidated on branch change, abort, compaction, dispose, and any applied refinement. Pinning a plan while a serialized background plan is in flight re-stamps the pin after the standard invalidation bump, so an accepted pin survives to its turn-boundary apply. The serialized-refine path resolves a pinned plan through the existing background-plan rail; the existing baselineState conflict rejection guards state that changed between preview and apply.

Not covered here, intentionally: gating auto-refine or agent-initiated refine.run() through a preview. A natural follow-up is a refineProposalReviewer hook on AgentSessionConfig (the post-plan sibling of #201's pre-plan autoRefineReviewer) called between plan and apply; serializeRefinementPlan() and the held-plan apply added here are built to be reused by it. Happy to take that on if there's interest.

Validation:

• npm run check
• npx tsx ../../node_modules/vitest/dist/cli.js --run test/refinement.test.ts
• npx tsx ../../node_modules/vitest/dist/cli.js --run test/suite/agent-session-refine-skill.test.ts
• npx tsx ../../node_modules/vitest/dist/cli.js --run test/suite/agent-session-serialized-refine.test.ts
• npx tsx ../../node_modules/vitest/dist/cli.js --run test/suite/agent-session-queue.test.ts
• npx tsx ../../node_modules/vitest/dist/cli.js --run test/suite/serialized-refine-config-integration.test.ts

Note

Add refine.preview endpoint and plan_id pinning to refine.run in coding agent

• Introduces refine.preview host request handler in agent-session.ts that runs the refinement planning pass and returns a serialized plan without applying it, caching up to 5 previewed plans per branch.
• Adds plan_id parameter to refine.run so a previously previewed plan can be applied exactly without re-planning; serialized mode short-circuits background planning when a pinned planId is present.
• Adds a serializeRefinementPlan utility in refinement.ts that produces snake_case kernel-facing payloads for previewed plans.
• Exposes the new refine.preview skill function in the Python skill layer via refine/__init__.py.
• Risk: applying an unknown or branch-invalidated plan_id throws an error rather than falling back to re-planning.

^{Macroscope summarized 3dec9ad.}