feat(index): worktree-aware index seeding

[aeneasr]

Summary

• Git worktree support: When git worktree add or claude --worktree creates a new worktree, Lumen detects sibling worktree indexes and copies the most recent one as a seed before running the normal Merkle tree diff
• Incremental instead of full re-index: Worktrees typically share 95%+ of files, so seeding turns minutes of embedding into seconds of incremental update
• Zero configuration: Detection, seeding, and incremental update are fully automatic

New packages

Package	Purpose
internal/git	Worktree detection: IsWorktree, CommonDir, ListWorktrees
internal/config	FindDonorIndex searches siblings for existing indexes
internal/index	SeedFromDonor does atomic SQLite copy with WAL checkpoint

Integration points

• cmd/stdio.go: Seeds before NewIndexer in getOrCreate() — runs only when DB doesn't exist yet
• cmd/hook.go: Reports sibling availability in SessionStart context so the user knows fast re-index is available
• README.md: Documents git worktree support in "What you get" and "Database location"

Edge cases handled

• git not installed → gracefully skipped
• Concurrent writes to donor → WAL checkpoint + file copy is safe
• Stale donor → Merkle diff catches all differences
• macOS /var → /private/var symlinks → resolved via EvalSymlinks

Test plan

• internal/git: IsWorktree with .git dir vs file, ListWorktrees with real repo, CommonDir, not-a-repo graceful failure
• internal/config: FindDonorIndex with no siblings, with sibling, wrong model mismatch
• internal/index: SeedFromDonor copies DB, skips when dst exists, incremental update after seed works correctly
• go test ./... — all packages pass
• golangci-lint run — 0 issues

🤖 Generated with Claude Code