feat(coding-agent): add provider payload hook

packages/coding-agent/docs/extensions.md

@@ -243,6 +243,7 @@ user sends prompt ────────────────────
   │   │                                            │       │
   │   ├─► turn_start                               │       │
   │   ├─► context (can modify messages)            │       │
+  │   ├─► before_provider_request (can inspect or replace payload)
   │   │                                            │       │
   │   │   LLM responds, may call tools:            │       │
   │   │     ├─► tool_call (can block)              │       │
@@ -489,6 +490,21 @@ pi.on("context", async (event, ctx) => {
 });
 ```
 
+#### before_provider_request
+
+Fired after the provider-specific payload is built, right before the request is sent. Handlers run in extension load order. Returning `undefined` keeps the payload unchanged. Returning any other value replaces the payload for later handlers and for the actual request.
+
+```typescript
+pi.on("before_provider_request", (event, ctx) => {
+  console.log(JSON.stringify(event.payload, null, 2));
+
+  // Optional: replace payload
+  // return { ...event.payload, temperature: 0 };
+});
+```
+
+This is mainly useful for debugging provider serialization and cache behavior.
+
 ### Model Events
 
 #### model_select
@@ -1934,6 +1950,7 @@ All examples in [examples/extensions/](../examples/extensions/).
 | `dirty-repo-guard.ts` | Warn on dirty git repo | `on("session_before_*")`, `exec` |
 | `input-transform.ts` | Transform user input | `on("input")` |
 | `model-status.ts` | React to model changes | `on("model_select")`, `setStatus` |
+| `provider-payload.ts` | Inspect or patch provider payloads | `on("before_provider_request")` |
 | `system-prompt-header.ts` | Display system prompt info | `on("agent_start")`, `getSystemPrompt` |
 | `claude-rules.ts` | Load rules from files | `on("session_start")`, `on("before_agent_start")` |
 | `file-trigger.ts` | File watcher triggers messages | `sendMessage` |