docs(provider): memory-plane v1 contract (basic plane) + ignore provider runtime DBs

- docs/contracts/bifrost_memory_provider.contract.md: v1 memory consumer spec —
  SQLite+sqlite-vec basic plane (describe_store / search / get / upsert / delete),
  honest capability advertisement (edges/atomic/transaction off, optimistic-lock on),
  affect-reused replay-or-conflict idempotency, #195 parity gate. Memory is a
  STRUCTURAL INDEX (reads vector/scope/id/origin), not a blind conduit (INV-001).
- .gitignore: *.db (+ wal/shm) — provider stores hold persisted agent affect/memory
  state; never track them.

.gitignore

@@ -116,3 +116,8 @@ Thumbs.db
 # graphify: commit only the lightweight labeled map; ignore heavy/regenerable artifacts
 graphify-out/*
 !graphify-out/GRAPH_REPORT.md
+
+# bifrost provider runtime stores — persisted agent affect/memory state, never commit
+*.db
+*.db-shm
+*.db-wal