refactor: finish low-tail tool Effect migration

[Astro-Han]

Summary

Finish the low-priority tool tail in the #936 Effect migration checklist: question.ts, todo.ts, invalid.ts, and plan.ts.

Changes:

• Added shared Effect-harness execute coverage for QuestionTool, TodoWriteTool, InvalidTool, and PlanExitTool.
• Moved PlanExitTool's last-model scan behind a named Effect boundary and switched its synthetic build-agent timestamp to Clock.currentTimeMillis.
• Updated packages/opencode/specs/effect-migration.md to mark the four tool checklist items complete.

Why

The tool bodies were already mostly Effect-native, but the checklist still had four low-tail items open and there was no single focused test proving their init/execute paths through the current Effect tool framework. This closes that tail without touching HTTP/server, remote, UI, desktop, or registry behavior.

Related Issue

Related to #936

Human Review Status

Pending

Review Focus

Please focus on whether the new tool-tail-effect.test.ts coverage is the smallest useful public-interface proof for these tools, and whether the PlanExitTool Clock.currentTimeMillis change preserves the existing handoff behavior.

Risk Notes

Runtime risk is low: the only production code change is plan_exit moving a synchronous model-history scan into Effect.sync and reading the timestamp from the Effect clock. The migration spec update is factual for this PR only.

Skipped checklist items:

• Visible UI/manual check: not applicable; no visible UI or user-facing copy changed.
• Platform/packaging check: not applicable; no platform, packaging, updater, signing, path, shell, or permission surface changed.

How To Verify

bun install --frozen-lockfile: passed
bun test test/tool/question-decoder.test.ts test/tool/external-result.test.ts test/tool/external-result-registry.test.ts test/tool/tool-define.test.ts test/tool/registry.test.ts test/session/todo.test.ts: baseline passed, 94 tests
bun test test/tool/tool-tail-effect.test.ts: RED on plan_exit Date.now vs Effect clock, then passed after the Clock.currentTimeMillis change, 4 tests
bun test test/tool/tool-tail-effect.test.ts test/tool/question-decoder.test.ts test/tool/external-result.test.ts test/tool/external-result-registry.test.ts test/tool/tool-define.test.ts test/tool/registry.test.ts test/session/todo.test.ts: passed, 98 tests
GOMAXPROCS=2 bun run typecheck: passed
git diff --check: passed

Screenshots or Recordings

Not applicable. No visible UI changes.

Checklist

How to use this checklist:

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• Human Review Status above is set to Pending, Approved by @<reviewer>, or Not required: <reason> (default is Pending; "not required" is restricted to bot-authored low-risk PRs).
• I linked the related issue, or stated in Summary why there is no issue.
• I described the review focus and any meaningful risks.
• I replaced the example block in How To Verify with the real verification steps and the key result for each.
• I did not introduce unrelated refactors, dependencies, generated files, or file changes beyond the stated scope.
• (conditional) I manually checked visible UI or copy changes when needed, with screenshots or recordings. Leave unticked only if no visible UI or copy changed.
• (conditional) I considered macOS and Windows impact for platform, packaging, updater, signing, paths, shell, or permissions changes. Leave unticked only if no platform/packaging surface was touched.
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• I reviewed the final diff for unrelated changes and suspicious dependency changes.
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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 6a78ae15-e7f8-46b4-b27d-6fae28b4e1bf

📥 Commits

Reviewing files that changed from the base of the PR and between 63122e6 and 30eed97.

📒 Files selected for processing (3)

• packages/opencode/specs/effect-migration.md
• packages/opencode/src/tool/plan.ts
• packages/opencode/test/tool/tool-tail-effect.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/i936-tool-tail-effect

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[github-actions]

Suggested priority: P2 (includes non-doc, non-test paths outside the low-risk bucket).

P1/P0 are reserved for maintainer confirmation. Please relabel manually if this is a release blocker, security issue, data-loss risk, or updater/runtime failure.