fix(mcp): basename source_file in tool_get_drawer responses

The MCP `mempalace_get_drawer` tool returned the entire raw drawer
metadata blob to any connected client, and the `source_file` field
in that blob is the absolute filesystem path written by the miners
(`miner.py`, `convo_miner.py` — `source_file = str(filepath)`). On
a single-user local deployment this is self-disclosure, but in
nested-agent or multi-server MCP topologies the client is a separate
trust domain and the host's directory layout has no documented
client-side use.

Mirror the mitigation that `searcher.search_memories()` already applies
on its own return path: reduce `source_file` to its basename via
`Path(source_file).name` before handing the metadata to the client.
Citations still work — the directory layout does not leak.

Companion to #1 (omit palace_path from tool_status). Same threat class,
different surface:

- mempalace_status — palace dir path     → fixed in #1
- mempalace_get_drawer — per-drawer source_file path → this PR

Other read tools were audited and do not leak host paths:
- mempalace_search    — already basenames source_file
- mempalace_list_drawers — returns wing/room/preview only
- mempalace_diary_read   — date/timestamp/topic/content only
- mempalace_reconnect    — success/message/drawers only
- mempalace_kg_*         — entity/predicate strings, counts
- mempalace_check_duplicate — wing/room/preview only

Changes:
- mempalace/mcp_server.py: tool_get_drawer() now basenames metadata.source_file
- tests/test_mcp_server.py: regression test asserting the absolute path
  and its parent directory do not appear anywhere in the response
- website/reference/mcp-tools.md: clarify the documented return shape

mempalace/mcp_server.py

@@ -912,12 +912,21 @@ def tool_get_drawer(drawer_id: str):
             return {"error": f"Drawer not found: {drawer_id}"}
         meta = result["metadatas"][0]
         doc = result["documents"][0]
+        # source_file is the absolute filesystem path written by the
+        # miners. Reduce to its basename before handing it to the MCP
+        # client — same threat model as the palace_path leak fix:
+        # nested-agent / multi-server topologies treat the client as a
+        # separate trust domain. Basename preserves citation utility.
+        # Mirrors the searcher.search_memories() return shape.
+        safe_meta = dict(meta) if meta else {}
+        if safe_meta.get("source_file"):
+            safe_meta["source_file"] = Path(safe_meta["source_file"]).name
         return {
             "drawer_id": drawer_id,
             "content": doc,
-            "wing": meta.get("wing", ""),
-            "room": meta.get("room", ""),
-            "metadata": meta,
+            "wing": safe_meta.get("wing", ""),
+            "room": safe_meta.get("room", ""),
+            "metadata": safe_meta,
         }
     except Exception as e:
         return {"error": str(e)}