fix: one tool-set recipe per session, so a tool cannot vanish between rounds

Two "unknown tool (not in this turn's tool set)" failures, one disease: the
turn's tool set was rebuilt from a different recipe depending on which entry
point happened to drive it.

A sub-agent got `ask_user_clarification`, `execute_subtask` and `activate_tools`
and nothing else — while `agents/common/tools.md` and every reporting agent's
prompt tell it to register its output with `update_scratchpad`. The child could
see the scratchpad injected into its context but had no way to write to it. It
now gets the scratchpad and todos tools, on the parent's `scratchpad_sid`: one
blackboard per session, as the surrounding code already declared.

`show_file_to_user` was injected per message by the WS handler, while
`resume_session` and `resolve_pending_call` rebuilt the list with `execute_task`
alone. So approving a card, or reconnecting mid-turn, continued the *same*
conversation with the tool silently gone. There is now a single recipe,
`ChatHub::session_interface_tools`, used by all three paths and fed by a builder
the shell installs once through `Skald::set_interface_tools_builder`: the core
keeps owning the tool, the shell keeps owning the policy of who gets it —
Telegram still does not, since it cannot act on OpenFile.

crates/skald-core/src/skald/accessors.rs

@@ -63,6 +63,16 @@ impl Skald {
 
     fn rt_user_contexts(&self) -> &super::user_context::UserContextRegistry { &self.user_contexts }
 
+    /// Declares the tools the running surface contributes to a chat session
+    /// (the SPA's `show_file_to_user`, …). Called once by the shell after
+    /// construction: the core owns the tools, the shell owns the policy of who
+    /// gets them. Every per-user hub built from here on receives it, and every
+    /// path that starts or resumes a turn consults it — see
+    /// [`crate::chat_hub::InterfaceToolsBuilder`].
+    pub fn set_interface_tools_builder(&self, build: crate::chat_hub::InterfaceToolsBuilder) {
+        self.rt_user_contexts().set_interface_tools_builder(build);
+    }
+
     /// The user's runtime context IF it is already live (built), **without**
     /// building one — used to refresh a logged-in user in place. A user who never
     /// logged in has no snapshot to refresh; their next login builds a fresh one.