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/main.py

@@ -19,6 +19,7 @@ from lembas.api import (
     admin_prompts,
     admin_search,
     admin_suggestions,
+    admin_tools,
     admin_users,
     audio,
     auth,
@@ -120,6 +121,7 @@ def create_app() -> FastAPI:
     app.include_router(admin_search.router)
     app.include_router(admin_prompts.router)
     app.include_router(admin_suggestions.router)
+    app.include_router(admin_tools.router)
 
     register_error_handlers(app)
     return app