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
* feat(coding-agent): add input event for extension input interception
Extensions can now intercept, transform, or handle user input before the
agent processes it. Three result types: continue (pass through), transform
(modify text/images), handled (respond without LLM). Handlers chain
transforms and short-circuit on handled. Source field identifies origin.
* fix: make source public, use if/else over ternary
* fix: remove response field, extension handles own UI
