fix(doctor): run PGLite probes through active engine

[duncanclaw]

Summary

Fix two false-positive gbrain doctor warnings for PGLite-backed brains:

• route the pgvector probe through the active BrainEngine
• route the jsonb_integrity probe through the active BrainEngine
• add focused PGLite regression coverage for both checks

Root Cause

The doctor had two remaining direct calls to the module-level Postgres singleton:

• db.getConnection() for pg_extension
• db.getConnection().unsafe(...) for JSONB integrity scanning

That works for Postgres-backed brains, but PGLite uses PGLiteEngine and does not initialize the Postgres singleton. The result was a false warning on healthy embedded brains:

[WARN] pgvector: Could not check pgvector extension
[WARN] jsonb_integrity: Could not check JSONB integrity

The underlying PGLite engine can answer both SQL probes correctly; the doctor was just bypassing it.

Real Behavior Proof

• Behavior addressed: embedded PGLite brains no longer warn on pgvector and jsonb_integrity solely because no external Postgres singleton is connected.
• Real environment tested: local GBrain checkout with an embedded PGLite engine, plus a PGLite-backed brain used for doctor smoke verification.
• Exact commands run after this patch:

bun test test/doctor.test.ts
bun run typecheck
bash scripts/check-no-legacy-getconnection.sh
bun run check:jsonb
gbrain doctor

• Evidence after fix:

(pass) doctor command > pgvector and jsonb_integrity checks use the active PGLite engine
49 pass
0 fail

[OK] pgvector: Extension installed
[OK] jsonb_integrity: All JSONB columns store objects/arrays

• Observed result after fix: the two PGLite-specific false warnings are gone. The local doctor smoke still surfaced an unrelated resolver warning from the checkout, but pgvector and jsonb_integrity both report OK.
• What was not tested: live Supabase/self-hosted Postgres doctor output. The SQL and status semantics are unchanged for Postgres; this PR only routes the probes through the active engine abstraction.

Regression Test Plan

• Coverage level that should have caught this: focused doctor unit coverage against a real PGLite engine.
• Target test: test/doctor.test.ts
• Scenario locked in: pgvectorCheck(PGLiteEngine) and jsonbIntegrityCheck(PGLiteEngine) both return OK after schema initialization.
• Why this is the smallest reliable guardrail: the bug was not pgvector or JSONB behavior; it was the doctor using the wrong connection path.

Verification

• bun test test/doctor.test.ts
• bun run typecheck
• bash scripts/check-no-legacy-getconnection.sh
• bun run check:jsonb

User-visible Behavior

PGLite users should see fewer false gbrain doctor warnings. No CLI flags, config keys, migrations, or data-shape changes.