refactor(agents): split tool registry into pure-data catalog, decouple connectors

Replace the connector-coupled BUILTIN_TOOLS registry with a pure-data
catalog so shared/tools no longer imports any connector module, making the
connector packages independently deletable.

- add shared/tools/catalog.py (ToolMetadata + TOOL_CATALOG, 41 tools, no imports)
- point GET /agent/tools (the only live consumer) at the catalog
- relocate ToolDefinition into action_log middleware (its sole consumer);
  drop the inert tool_definitions wiring (no tool defines reverse)
- delete shared/tools/registry.py: connector imports, dead factories,
  dead get_connector_gated_tools, and BUILTIN_TOOLS
- drop stale dedup-propagation test (path removed in C1) + refresh docstrings

import-all guardrail + agents unit suite green (987 passed).

surfsense_backend/app/agents/shared/agent_cache.py

@@ -113,12 +113,11 @@ def tools_signature(
       MCP tools loaded for the user changes, gating rules flip, etc.).
     * The available connectors / document types for the search space
       change (new connector added, last connector removed, new document
-      type indexed). Because :func:`get_connector_gated_tools` derives
-      ``modified_disabled_tools`` from ``available_connectors``, the
-      tool surface is technically already covered — but we hash the
-      connector list separately so an empty-list "no tools changed"
-      situation still rotates the key when, say, the user re-adds a
-      connector that gates a tool we were already not exposing.
+      type indexed). Connector gating derives disabled tools from
+      ``available_connectors``, so the tool surface is technically already
+      covered — but we hash the connector list separately so an empty-list
+      "no tools changed" situation still rotates the key when, say, the user
+      re-adds a connector that gates a tool we were already not exposing.
 
     Stays stable across: