feat: add greplica graph gc to prune stale knowledge-graph memory

[burrows99]

Closes #24.

greplica graph gc [--dry-run] prunes memory a repo no longer supports.

What it detects

• Stale anchors — components/claims whose code_anchor file no longer exists on disk. Reuses the existing code-anchor audit (auditClaimCodeAnchors, already behind graph audit anchors) for claims, and adds the same file check for component anchors.
• Orphaned claims/flows — no edge references them.
• Dangling edges — an endpoint object row no longer exists.

Prune semantics

• Pruning a subject cascades to every edge that touches it (a stale claim's about/evidenced_by edges go with it), all in a single transaction.
• Object rows are global across repos (bug: Graph object IDs are global across repos, causing cross-repo memory collisions #103), so gc is scope-safe: it only unlinks this repo's graph_memberships + repo-scoped graph_object_embeddings, and deletes the shared object row only when no membership anywhere still references it. No cross-repo damage.
• Superseded objects are left alone (supersession is history).
• --dry-run reports the plan and touches nothing.

Design note (the open question from the thread)

The issue says "permanently prunes," and #103 makes a naive global DELETE dangerous. This PR reconciles both: hard-delete, but gated on no remaining references, so it reclaims rows without affecting other repos. Happy to switch to a pure retire-via-supersession model if maintainers prefer.

Tests

scripts/check-graph-gc.js (wired into npm test) seeds a temp DB with one of each defect class plus healthy objects, then asserts: dry-run counts + zero mutation, real run prunes exactly the defects with cascaded edges/memberships/embeddings, healthy objects untouched, and a second pass is a no-op. npm test and npm run typecheck both pass.

Demo

Recording being attached in a comment.

[burrows99]

Full graph gc walkthrough covering every behavior #24 asks for. One graph is seeded with all the rot at once: a stale code anchor (we delete src/cache.ts), an orphaned claim and an orphaned flow (no edges), and a dangling edge (points at a deleted subject). graph gc --dry-run reports all of it without touching the database; graph gc then prunes it (cascading edges, memberships, embeddings) and leaves only the healthy claim.

gc-full.mp4