feat(pg): add switchable PostgreSQL backend + OpenClaw/Hermes memory bridge
Add an optional PostgreSQL backend (QMD_BACKEND=pg) alongside the
unchanged default SQLite path. PG store uses pgvector (HNSW) for vectors
and pg_jieba + pg_trgm for full-text/Chinese tokenization, with a
namespace column isolating multi-agent memory (openclaw/hermes).
- src/pg/: config, db-pg, schema bootstrap, memory store
- MCP memory_add/memory_search/memory_get tools; qmd pg status + memory CLI
- connection via QMD_PG_URL/DATABASE_URL/qmd config, stunnel TLS 5443
- tests: pg-config (unit) + pg-memory integration (gated on QMD_PG_URL) + pg-compose
- docs/plan: plan, usage, test report, changelog; track docs/**/*.md
SQLite path: zero regression (typecheck clean, 249 passed / 6 skipped).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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