fix(pi-claude-cli): unblock parameterless MCP tool calls in triage

Triage with claude-sonnet-4-6 via pi-claude-cli kept looping on
fn_review_spec calls that were rejected by pi's validator with
"root: must be object". Parameterless MCP tools (schema
{type:"object", properties:{}}) emit zero input_json_delta events,
so partialJson stayed "" and the catch fell through to
finalArgs = "" — a string, which TypeBox's Type.Object({}) rightly
refuses. Default empty partialJson to {} so the call lands.

Also:
- Add a 2-step reminder loop in triage before swapping to the
  fallback planning model — primary models that wrote PROMPT.md
  but forgot fn_review_spec recover from a nudge, no need to pay
  the cold-start tax of a new triage on a different model.
- Inject @runfusion/fusion's own pi extension into dashboard/
  daemon/serve sessions and propagate the path to createFnAgent
  via setHostExtensionPaths so fn_* tools register globally
  without requiring `pi install npm:@runfusion/fusion`.
- Drop the "historical" qualifier from replayed tool labels —
  Claude was reading "TOOL RESULT (historical Read):" as
  "previous session, ignore" and looping on verification.
- Remove subprocess-lifecycle stderr debug logs that landed for
  hang diagnosis — root cause is fixed, the noise can go.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/src/pi.ts

@@ -50,6 +50,24 @@ export interface AgentResult {
   sessionFile?: string;
 }
 
+/**
+ * Process-global list of extension paths to inject into every createFnAgent
+ * session. Set once at startup by the host (cli's dashboard/daemon/serve)
+ * before any sessions are created. The paths are passed to pi's
+ * `DefaultResourceLoader` as `additionalExtensionPaths` so the cli's own
+ * `@runfusion/fusion` extension (registering `fn_*` tools) is loaded inside
+ * every agent session — including chat sessions that pass no `customTools`.
+ */
+let hostExtensionPaths: string[] = [];
+
+export function setHostExtensionPaths(paths: readonly string[]): void {
+  hostExtensionPaths = [...paths];
+}
+
+export function getHostExtensionPaths(): readonly string[] {
+  return hostExtensionPaths;
+}
+
 export interface PromptableSession extends AgentSession {
   promptWithFallback: (prompt: string, options?: unknown) => Promise<void>;
 }
@@ -1066,6 +1084,10 @@ export async function createFnAgent(options: AgentOptions): Promise<AgentResult>
     settingsManager,
     systemPromptOverride: () => options.systemPrompt,
     appendSystemPromptOverride: () => [],
+    // Inject host-supplied extension paths (e.g. cli's own `@runfusion/fusion`
+    // extension that registers `fn_*` tools) so they're loaded inside every
+    // agent session, including chat sessions that don't pass `customTools`.
+    ...(hostExtensionPaths.length > 0 ? { additionalExtensionPaths: [...hostExtensionPaths] } : {}),
     ...(skillsOverrideFn ? { skillsOverride: skillsOverrideFn } : {}),
   });
   await resourceLoader.reload();