Compaction: make extractTailAfterSummary summary-aware

[riatzukiza]

Follow-up from PR #20 review (CodeRabbit comment on tail extraction): #20 (comment)

Current behavior in lib/compaction/codex-compaction.ts::extractTailAfterSummary returns a cloned slice starting at the last role:"user" message. That matches the existing compaction flow, but if we ever keep summaries as user messages in the client-side transcript, they could be mistaken for the tail start.

This issue tracks:

• Adding an explicit way to mark summary items (e.g., metadata flag or dedicated field) so they are not treated as the conversation tail.
• Updating extractTailAfterSummary (and any related helpers) to skip summary-marked items when finding the tail start.
• Adjusting tests in test/codex-compaction.test.ts to clearly cover the summary-aware behavior.

We intentionally deferred this refinement from PR #20 to keep the change set focused.

[riatzukiza]

Triage: to make extractTailAfterSummary summary-aware, propose: (1) introduce a summary marker (metadata flag or dedicated field) on transcript items; (2) update extractTailAfterSummary to skip summary-marked items when finding the tail; (3) adjust tests for both marked and unmarked transcripts, keeping backward compatibility. Please confirm the preferred marker shape (metadata key vs field) before I implement.

[riatzukiza]

Closing as obsolete. Plugin-level compaction (including extractTailAfterSummary) was removed from the codebase; no compaction code remains (rg finds none). We can revisit summary-aware tail extraction if/when compaction is reintroduced with a new design.