fix(session): reconcile compaction summary with preserved tail

[samiralibabic]

Issue for this PR

Closes #38530

Replaces #28067, which was auto-closed for template compliance without maintainer review.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Repeated compaction can retain stale work under Next Move when the previous summary lists an action and the preserved recent turns show it was completed. The summarizer currently cannot reconcile that evidence because the preserved tail is excluded from its input.

This supplies a bounded, transformed audit of visible user and assistant text from the preserved tail while keeping the normal tail replay unchanged.

How did you verify your code works?

• cd packages/opencode && bun test test/session/compaction.test.ts (57 pass, 1 skip)
• cd packages/opencode && bun run typecheck
• Changed-file Prettier and git diff --check
• Pre-push workspace typecheck (30 packages passed)

Screenshots / recordings

N/A. This is not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Related PRs Found

The search revealed several related PRs addressing session compaction issues:

1. feat(core): enhanced compaction with ratios and combined context-restoration advancement #45125 - feat(core): enhanced compaction with ratios and combined context-restoration advancement

   • Likely related as it addresses enhancements to compaction logic

2. fix(session): bound compacted history hydration #39930 - fix(session): bound compacted history hydration

   • Directly related - also addresses session compaction and history handling

3. fix(opencode): reject empty compaction summaries #42063 - fix(opencode): reject empty compaction summaries

   • Related - addresses compaction summary validation

These appear to be part of an ongoing effort to improve session compaction behavior. However, #46364 (the current PR) is specifically addressing the reconciliation of compaction summaries with preserved tail contents, which is a distinct issue from the others even if they touch related code areas.

The PR also references #28067 (auto-closed without review), which this PR replaces.

[github-actions]

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