feat(llm-worker): record terminal observation coverage

[Xore]

Summary

• add bounded terminal-observation and capture-coverage values to SessionAccumulator
• preserve close, idle, and still-open distinctions without inferring a termination reason
• expose covered/excluded session counts in cycle status and document the denominator
• add synthetic coverage tests and ambiguous-terminal-absence documentation

Validation

• python -m unittest discover -s llm-worker/tests -v (77 tests, OK)
• python llm-worker/worker.py --selftest (PASS)

Closes #3292

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[Xore]

Closing as already-landed. Rebasing onto main after #3302 merged shows this branch is empty: the terminal_observation / capture_coverage fields and the ambiguity wording from this PR are present on main, carried in by #3302 (offline engagement aggregate), which was built on this branch's schema and included it. The only conflict was this PR's doc/test sections that #3302 had already superseded. Verified on main: TERMINAL_OBSERVATION_CLOSE and capture_coverage both in llm-worker/worker.py; 82/82 llm-worker tests pass.