Session-state importer ΓÇö backfill from Copilot CLI session history

Summary

Build a session-state scanner that reads ~/.copilot/session-state/ to backfill brag entries from past Copilot CLI sessions.

What to build

lib/session-state.mjs ΓÇö session-state importer

Scan ~/.copilot/session-state/{id}/ directories and extract:

• workspace.yaml ΓåÆ summary, CWD, git root, repo, branch, created_at
• events.jsonl ΓåÆ lifecycle timestamps (session.start, session.resume, session.shutdown), model, token usage, duration

Create session records from discovered data with deduplication.

New tool: backfill_copilot_sessions

Scan my copilot session history and create brag entries for the significant ones.

Enhanced orphan recovery

• Use events.jsonl timestamps to identify crashed/orphaned sessions
• Salvage useful metadata into brag entries
• Generate summaries from session context

Key data sources

Files to create/modify

• New: lib/session-state.mjs, test/session-state.test.mjs
• Modify: extension.mjs (new tool), docs/backfill-guide.md

Estimated effort

• MVP scanner + tool: 2ΓÇô3 days
• Tests + docs: 1 day

Start with the existing tool registration in extension.mjs and inspect the documented session-state paths and fields in the issue. Implement and test lib/session-state.mjs against workspace.yaml and events.jsonl, including deduplication and orphan recovery, then run test/session-state.test.mjs. Done means the new backfill_copilot_sessions tool creates useful session records and docs/backfill-guide.md explains the workflow.