Fix archived chats remaining in the history panel - #13

Summary

Archived conversations can remain in session_index.jsonl, so the Codex Chats panel continues to show them after they are archived.

• Exclude indexed sessions whose IDs appear in archived_sessions, reading filenames only and supporting nested directories and compressed .jsonl.zst archives.
• Refresh on archive creation/deletion using scoped VS Code file system watchers, including creation or replacement of the archive directory itself. Coalesce event bursts with a 100 ms debounce; do not poll the archive.
• Add regression coverage for filtering, archiving/restoring without index changes, watcher scope, event coalescing, and disposal. Update README and changelog without changing the extension version.

Validation

• Windows / Node.js 24: npm test — 19 tests passed; npm run package succeeded.
• Linux (Ubuntu/WSL) / Node.js 22.23.3: npm test — 19 tests passed; npm run package -- --out codex-extras-ci.vsix succeeded. Dependencies were installed from the local npm cache with --offline --ignore-scripts because WSL DNS was unavailable.
• Confirmed the filtering regression fails against the original implementation and passes with this fix.
• Verified the packaged VSIX includes session-archive.js.
• git diff --check passed.

Checklist

• The change is focused and contains no unrelated files.
• npm test passes.
• npm run package succeeds.
• User-facing behavior is documented.
• CHANGELOG.md is updated when appropriate.
• No credentials, personal data, or generated build artifacts are committed.