feat(hoosage): cache session indexes across windows

[wakemeup0]

Every new VS Code window re-read all saved Copilot usage, which kept the dashboard on "Indexing saved usage…" for seconds on each start:

• Copilot CLI session files (Windows plus WSL over 9P, in 4 MB batches per poll)
• Chat transcripts (a full re-parse)

This PR adds a local index cache under globalStorageUri/cache/.

CLI / JetBrains / WSL session-state

• Stores the byte offset, inode, reader state and usage read so far for each file.
• A later window reads only the bytes appended since then.
• Line splitting is now byte-based, so the resume offset is exact, including for partial UTF-8 lines.
• Sessions whose directory disappeared are dropped.

Chat transcripts

• Stores size, mtime, projectId and allowlisted usage for each file.
• Unchanged transcripts are not parsed again.

Startup

• Previously imported chat-history.json usage is shown immediately, before the rescan.

Safety

• The cache is keyed by the extension version, so it is rebuilt once per update.
• Each file entry is validated. A missing, corrupt, truncated or replaced source falls back to a full read.
• Writes are atomic (tmp + rename) and best-effort.
• Stopped WSL distros are still never touched.

Measured on a real profile (cold → warm)

Totals were identical with and without the cache.

Source	Cold	Warm
Windows CLI (53 files, 77 MB)	529 ms	17 ms
WSL CLI (28 files, 148 MB)	1927 ms (10 polls)	305 ms
Chat transcripts	1690 ms	95 ms

Tests

• npm run check: 80/80 tests, typecheck, build and calendar UI all pass.
• New test/scan-cache.test.ts covers:
  • resuming from the cache;
  • cached vs full reads matching, including a split UTF-8 line;
  • a key mismatch;
  • truncation;
  • removed sessions and a corrupt cache;
  • transcript cache hits and misses.
• npm run test:extension fails on Windows on main as well: pathToFileURL gives file:///C:/…, while VS Code uses file:///c%3A/….
  • With that locally patched, all activation, saved-usage, CLI-discovery and Chat-import assertions pass.
  • The remaining failure is the Copilot enable/status check on the current Insiders build and is unrelated to this change.