feat(zettel): add graph traversal agent tool

[adihex]

Summary

• add a validated tenant-aware traverseGraph agent tool backed by traverseGraphStore
• register the tool and minimally update the Zettel system prompt
• cover success, tenant/default forwarding, depth bounds, and store failures

Validation

• mise exec -- pnpm exec vitest run apps/zettel/src/tools/notes.test.ts apps/zettel/src/notes/store.test.ts
• mise exec -- pnpm exec vitest run apps/zettel/src (40 tests)
• mise exec -- pnpm build

[adihex]

Landed on main as part of the consolidated, rebased stack ending at e9bd4bd. Verified this PR has no patches absent from origin/main, so closing the now-redundant stacked PR.