feat(pegas-memory): Tier-1 self-improvement tools (commit + staleness)

- memory_commit: closes the 'commit hole' — writes verified answers from
  closed gaps back into atoms (mandatory dedup); leaves gap open if no
  verified answer exists. Reads verified status from PostgreSQL (source of
  truth), not the Qdrant projection.
- memory_stale: staleness guard with two signals — content-staleness
  (re-fetch + LLM verdict, needs proxy for foreign sites) and age-staleness
  (proxy-free fallback by pm_document.fetched_at, max_age_days).
- db: populate pm_document.fetched_at on upsert (freshness signal).
- docs: README/AGENTS/ARCHITECTURE/EXAMPLES updated (18 tools).
- self_improve.py: new module (227 lines).

ARCHITECTURE.md

@@ -1,7 +1,7 @@
 # Pegas Memory — Architecture
 
 Pegas Memory is the long-term memory and knowledge base for the **Pegas** research agent.
-It is a Python service exposed to the agent as an **MCP server** (16 tools).
+It is a Python service exposed to the agent as an **MCP server** (18 tools).
 
 ---
 
@@ -14,7 +14,7 @@ It is a Python service exposed to the agent as an **MCP server** (16 tools).
                  └───────────────────┬──────────────────────┘
                                      │  JSON-RPC 2.0 / stdio
                  ┌───────────────────▼──────────────────────┐
-                 │  mcp_server.py  (tool layer, 16 tools)   │
+                 │  mcp_server.py  (tool layer, 18 tools)   │
                  └───────────────────┬──────────────────────┘
                                      │  Python API
         ┌────────────────────────────┼────────────────────────────┐