feat(coding-agent): expose streamingBehavior on InputEvent

Add streamingBehavior to InputEvent so extensions can distinguish
idle prompts from mid-stream steers and queued follow-ups.

- Add streamingBehavior field to InputEvent type
- Thread it through ExtensionRunner.emitInput() and AgentSession.prompt()
- Add streaming-aware input gate example with tests
- Document in extensions.md

packages/coding-agent/docs/extensions.md

@@ -819,6 +819,9 @@ pi.on("input", async (event, ctx) => {
   // event.text - raw input (before skill/template expansion)
   // event.images - attached images, if any
   // event.source - "interactive" (typed), "rpc" (API), or "extension" (via sendUserMessage)
+  // event.streamingBehavior - "steer" | "followUp" | undefined
+  //   undefined when idle, "steer" for mid-stream interrupts,
+  //   "followUp" for messages queued until the agent finishes
 
   // Transform: rewrite input before expansion
   if (event.text.startsWith("?quick "))
@@ -847,7 +850,7 @@ pi.on("input", async (event, ctx) => {
 - `transform` - modify text/images, then continue to expansion
 - `handled` - skip agent entirely (first handler to return this wins)
 
-Transforms chain across handlers. See [input-transform.ts](../examples/extensions/input-transform.ts).
+Transforms chain across handlers. See [input-transform.ts](../examples/extensions/input-transform.ts) and [input-transform-streaming.ts](../examples/extensions/input-transform-streaming.ts) for `streamingBehavior`-aware routing.
 
 ## ExtensionContext
 
@@ -2543,6 +2546,7 @@ All examples in [examples/extensions/](../examples/extensions/).
 | `confirm-destructive.ts` | Confirm session changes | `on("session_before_switch")`, `on("session_before_fork")` |
 | `dirty-repo-guard.ts` | Warn on dirty git repo | `on("session_before_*")`, `exec` |
 | `input-transform.ts` | Transform user input | `on("input")` |
+| `input-transform-streaming.ts` | Streaming-aware input transform | `on("input")`, `streamingBehavior` |
 | `model-status.ts` | React to model changes | `on("model_select")`, `setStatus` |
 | `provider-payload.ts` | Inspect payloads and provider response headers | `on("before_provider_request")`, `on("after_provider_response")` |
 | `system-prompt-header.ts` | Display system prompt info | `on("agent_start")`, `getSystemPrompt` |