Add opt-in chat sorting by latest delivered result

[priva7eer]

Summary

When several conversations run in parallel, session-index dates do not necessarily bring newly delivered results to the top of the chat list. Add optional codexExtras.chatSortOrder: latestResult, preserving the existing workspace groups and recent index-date sorting as the default.

Sort by the timestamp of the last explicit task_complete event. Opening, focusing, reading, starting work, questions, and streamed tokens cannot change the order. A newly completed result moves its conversation up only within its workspace group. Equal or missing result times use stable session IDs; undated results appear below dated ones.

• Cache reads of at most the final 256 KiB per session log and refresh via scoped, debounced VS Code file events.
• Remember observed completion times in extension storage, so a new turn or window reload cannot lose a previously seen result outside the log-tail budget.
• A completion outside that tail can be unknown on first observation; this limitation is documented.
• Setting changes apply immediately. Default mode keeps metadata-only reads; result sorting requires neither live IPC nor status icons and does not modify transcripts.

Independent of #13, #15, #16, and #17. No dependencies, version bump, or generated build artifacts.

Closes #18

Validation

• Windows / Node 24.14.0: npm test (20/20) and npm run package -- --out codex-extras-results.vsix.
• Ubuntu via WSL / Node 22.23.3: npm test (20/20) and npm run package -- --out codex-extras-status-ci.vsix. Dependencies installed from cache using --offline --ignore-scripts --no-audit --no-fund because WSL DNS was unavailable.
• Tests cover workspace boundaries, stable order after opening and work starts, a new completion moving a row, persistent completion times beyond the bounded tail after reload, default-sort restoration, watcher coalescing/disposal, malformed/partial records, truncation/deletion, and manifest defaults.
• The combined local development build also passes 52 tests on Windows and Linux. The independent PR excludes unrelated features.
• git diff --check passes.

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.