fix: bucket Codex usage by token_count deltas

[ericweichun]

Summary

• split Codex cumulative token_count snapshots into per-event deltas before aggregation
• keep duplicate-session handling by choosing the better complete session log, then flattening its delta entries
• fixes period buckets like Today / 7d / 30d so older cumulative usage does not leak into newer ranges

Test plan

• .venv/bin/python -m ruff check codex_loader.py tests/test_codex_loader.py
• .venv/bin/python -m mypy .
• .venv/bin/python -m pytest tests/test_codex_loader.py tests/test_analyzer_pipeline.py tests/test_menubar.py

Notes for reviewer

This keeps the rate-limit parser unchanged. It only changes the historical usage entries emitted from Codex session logs so downstream aggregators can bucket usage by each token_count event timestamp.

[aqua5230]

Thanks for the careful change. The semantic shift from cumulative to delta makes sense for accurate bucketing, but it
leaves v0.11.6's fast path inconsistent: analyzer/reporter.py:115-212 (_load_recent_codex_entries and
_parse_codex_file) still reads the final cumulative snapshot and uses session-start time as the entry timestamp, so
today/week/month reports will diverge from what the popover shows once your new loader lands.

Could you:

1. Either reuse codex_loader's new delta logic in the reporter fast path, or update _load_recent_codex_entries to
   bucket by token_count event timestamp the same way?
2. Add a reporter-layer test proving today/week/month reports pick up the deltas (not just the loader-level test)?

Happy to merge once these are in.

[ericweichun]

Updated, thanks. I rebased onto current main and removed the separate reporter Codex cumulative fast parser. The reporter recent path now reuses codex_loader.load_entries(hours_back=...), so today/week/month reports use the same token_count delta logic as the popover. I also added a reporter-layer today test that exercises a cross-day cumulative Codex session and verifies only the current-day delta is counted.\n\nValidation run locally:\n- .venv/bin/python -m pytest tests/test_codex_loader.py tests/test_analyzer_pipeline.py\n- .venv/bin/python -m ruff check analyzer/reporter.py tests/test_analyzer_pipeline.py\n- .venv/bin/python -m mypy .