fix(gbrain): cap agent/project fallback enrichment graph size (BLO-21635)

[allyblockcast]

Summary

• The gbrain-context recall prefetch (packages/plugins/paperclip-plugin-gbrain/src/recall.ts) falls back to the agent's or project's hub node when the issue-scoped traverse_graph comes back empty or a single-node island.
• That hub node can be linked to thousands of unrelated fact-* nodes — BLO-21635 §A found ~96% of ok recalls carrying a median ~22,636 nodes / ~850KB stored (pg_column_size) / ~5.8MB decoded JSON, almost entirely unrelated fact-* nodes from the untargeted hub dump — roughly 6.6GB/week of decoded graph pushed through recall.
• mergeGraphs() now caps the fallback's contribution to MAX_ENRICHMENT_NODES (50) before merging into the cached recall, preferring nodes structurally closest to the hub (lowest traversal depth) when available, and drops edges that would dangle to trimmed nodes. The issue-scoped traversal is untouched — it's already relevance-scoped and safe to attach whole.

Test plan

• pnpm test in packages/plugins/paperclip-plugin-gbrain — 99/99 passing, including 2 new tests covering the array-shape and nodes/edges-shape oversized-fallback cases (existing small-fallback tests are unaffected since capping is a no-op under the limit).
• pnpm typecheck — clean.
• Post-merge: re-run BLO-21635's §A verifying-signal query (select pg_column_size(value_json), value_json->>'note' ... status='ok') over a fresh window and confirm median node count / stored size drop materially from the ~22,636 / ~850KB baseline.

https://paperclip.blockcast.net/BLO/issues/BLO-21635

[allyblockcast]

🔗 Paperclip issue: BLO-21635

[allyblockcast]

🔗 Paperclip issue: BLO-21635

[allyblockcast]

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[kkroo]

Superseded by #1070, which carries the same BLO-21635 head with a compliant PR template and is opened from a human-authored branch so Ally can review it.