Tests: clarify extractTailAfterSummary semantics in codex-compaction

[riatzukiza]

Follow-up from PR #20 review (CodeRabbit comment on compaction helper tests): #20 (comment)

In test/codex-compaction.test.ts, the tests around extractTailAfterSummary currently read as if they are verifying a "summary-aware" helper, but the implementation actually returns the tail slice starting at the last role:"user" message.

This issue tracks:

• Renaming/rewriting the extractTailAfterSummary tests so they explicitly describe the current semantics (tail after the last user message).
• Avoiding test names or comments that imply a specific "summary marker" prefix while the implementation is still last-user-based.
• Optionally, adding an additional test once we implement a dedicated summary marker (see also issue Compaction: make extractTailAfterSummary summary-aware #21) so test coverage remains clear if/when the helper becomes summary-aware.

We decided to keep the existing test wording in PR #20 and handle this clarification in a follow-up.

[riatzukiza]

Triage: tests read as summary-aware but helper currently returns tail from last role:"user". Plan: (1) rename/reword extractTailAfterSummary tests to state the actual behavior (tail after last user message); (2) add/mark a pending case or TODO for summary markers once implemented; no behavior change now. OK to proceed with test wording update?

[riatzukiza]

Closing as obsolete. Plugin-level compaction helpers/tests (extractTailAfterSummary) were removed; repo has no compaction references now. If compaction returns in a new design, we can add fresh tests aligned to that implementation.