agent-loop: root turn driven by the library kernel (phase 2)

ChatSessionHandler now runs the root turn on the agent-loop kernel
instead of run_agent_turn; sub-agents follow on the same kernel via
DelegateTool. The old loop stays for resume/recovery until phase 3.

agent-loop:
- DelegateTool + AgentCatalog/AgentProfile (full toolset override,
  per-child selector/assembler, frame-scoped get), StaticCatalog,
  FilteredToolSet; sync flow with sticky child_token; batch via the
  generic fan-out
- manager: start_loop skips the registry (children are not
  double-driving; they ride the parent's token tree); LoopParams gains
  selector/token overrides
- store: get_frame, get_call, set_call_extras; HistoryStore result text
  aligned to raw-stored semantics (projection formats)
- events: ApprovalRequired.request_id, AgentSpawned/Finished parent
  info; AskUserTool with_name + suggested_answers alias + Question.frame

skald-core (loop_adapters + handler):
- SkaldAssembler (byte-parity port of MessageBuilder's projection:
  scratchpad/summary/window, DTL Kimi/Anthropic injection, media, user
  coalescing, reasoning echo) + AgentSystemContext (prompt layers,
  substitutions, MCP list, shared folders, user profile)
- SkaldAgentCatalog (build_sub_agent_config port), SkaldHumanChannel,
  scratchpad/todos tools, execute_task sync/async alias,
  LegacyInterfaceTool, PendingLiveInput
- ApprovalGate: PendingWrite diffs via LoopEvent::Host (memory/disk
  routed like the fs-tools); SkaldWritePreviewHook for executed-write
  diffs; EventTranslator LoopEvent→ServerEvent (display meta, preview,
  FileChanged, AgentStart/Done, root-only Done/Truncated/Cancelled)
- handle_message: builds TurnParams and drives the kernel; resume of
  pending tools runs first (results belong to the previous turn);
  ChatEvent publication stays handler-side; /stop cancels the live loop
- ToolRegistry.get_tool/all_tools; def builders made pub(crate)

Full workspace suite green (179 skald-core, 34 agent-loop, adapters
incl.); two pre-existing doc-test failures fixed along the way.

crates/agent-loop/src/activation.rs

@@ -66,10 +66,19 @@ pub trait ToolActivator: Send + Sync {
 /// other — the defs re-read at the next round makes the new grants visible.
 pub struct ActivateToolsTool {
     activator: Arc<dyn ToolActivator>,
+    definition_override: Option<Value>,
 }
 
 impl ActivateToolsTool {
-    pub fn new(activator: Arc<dyn ToolActivator>) -> Self { Self { activator } }
+    pub fn new(activator: Arc<dyn ToolActivator>) -> Self {
+        Self { activator, definition_override: None }
+    }
+
+    /// Override the advertised definition (legacy parity).
+    pub fn with_definition(mut self, def: Value) -> Self {
+        self.definition_override = Some(def);
+        self
+    }
 }
 
 #[async_trait]
@@ -77,6 +86,9 @@ impl Tool for ActivateToolsTool {
     fn name(&self) -> &str { "activate_tools" }
 
     fn definition(&self) -> Value {
+        if let Some(def) = &self.definition_override {
+            return def.clone();
+        }
         json!({
             "type": "function",
             "function": {