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/web/templates/admin/_layout.html

@@ -43,6 +43,10 @@
           {{ icon("globe", "icon--sm") }}
           <span class="nav-item__label">Web search</span>
         </a>
+        <a class="nav-item {{ 'is-active' if section == 'tools' }}" href="/admin/tools">
+          {{ icon("link", "icon--sm") }}
+          <span class="nav-item__label">Tools</span>
+        </a>
         <a class="nav-item {{ 'is-active' if section == 'prompts' }}" href="/admin/prompts">
           {{ icon("sparkle", "icon--sm") }}
           <span class="nav-item__label">Prompts</span>
@@ -64,10 +68,6 @@
 
       <div class="nav-group">
         <div class="nav-group__label">Not yet built</div>
-        <span class="nav-item is-disabled">
-          {{ icon("gear", "icon--sm") }}
-          <span class="nav-item__label">Tools</span>
-        </span>
         <span class="nav-item is-disabled">
           {{ icon("server", "icon--sm") }}
           <span class="nav-item__label">Agents</span>