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>

tests/test_harness.py

@@ -54,6 +54,27 @@ def test_only_the_guidance_for_offered_tools_appears(db, owner):
     assert "Skills are procedures" not in text
 
 
+def test_a_custom_tools_guidance_appears_only_when_it_is_offered(db, owner):
+    """The row supplies the default, and the fragment is gated on the tool's own
+    family -- which is why the registry had to stop being a module constant."""
+    from lembas.db.models import CustomTool
+
+    db.add(
+        CustomTool(
+            slug="weather",
+            name="Weather",
+            description="Look up the weather.",
+            url_template="https://api.test/{{city}}",
+            guidance="- Check the weather rather than guessing at it.",
+        )
+    )
+    db.commit()
+
+    offered = tools_service.registry(db)["weather"].schema
+    assert "Check the weather" in harness.compose(db, owner, [offered])
+    assert "Check the weather" not in harness.compose(db, owner, _tools("web_search"))
+
+
 def test_the_memory_block_is_included_when_memory_is_offered(db, owner):
     memories_service.add(db, owner=owner, content="Prefers metric units.")
     text = harness.compose(db, owner, _tools("memory_add"))