Custom HTTP tools an administrator defines

A row in custom_tools becomes a ToolDef like any built-in, offered beside
the thirteen. The registry had to stop being an import-time constant for
that: `resolve_tools` now returns the schemas *and* the runners together,
carried to the loop on the ToolContext.

That closes a hole on the way. `run_tool` looked names up in the global
REGISTRY with no reference to what had been offered, so a model naming a
tool its chat was gated out of -- a family switched off, a permission the
reader lacks -- had it run anyway. The resolved set is now authoritative.

Arguments come from a model, so an argument may fill a hole but never move
the target: the scheme and host of a URL template are literal, values are
escaped for where they land, and the origin is pinned afterwards. Every
redirect hop is checked the way services/fetch.py checks one, and the
secret is dropped if a hop leaves the origin it was issued for.

Also fixes the tool-activity block claiming every library tool had
"searched the web", which it has done since the second family landed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/security/permissions.py

@@ -86,6 +86,22 @@ PERMISSION_DEFS: tuple[PermissionDef, ...] = (
         True,
         "Chat",
     ),
+    PermissionDef(
+        "tools.custom",
+        "Use custom tools",
+        "Let a model call the HTTP tools an administrator has defined. Which "
+        "ones depends on the groups each tool is restricted to.",
+        True,
+        "Chat",
+    ),
+    PermissionDef(
+        "tools.mcp",
+        "Use MCP servers",
+        "Let a model call tools from the MCP servers an administrator has "
+        "added. Which ones depends on the groups each server is restricted to.",
+        True,
+        "Chat",
+    ),
     PermissionDef(
         "audio.transcribe",
         "Dictate messages",