docs(bf-549b): verify stream-json reader spawn at PROMPT_INJECTED

Verify that the stream-json reader spawn call is correctly wired
at the PROMPT_INJECTED transition in the event loop callback.

Code is already implemented in src/session.rs:359-377 with:
- Phase transition detection
- Output format check (StreamJson only)
- Transcript path and start_offset capture
- Handle storage for cleanup on all exit paths
- Compiles without errors

notes/bf-549b.md

@@ -0,0 +1,54 @@
+# bf-549b: Stream-Json Reader Spawn Verification
+
+## Task
+Wire stream-json reader spawn at PROMPT_INJECTED transition.
+
+## Status
+**ALREADY IMPLEMENTED** - Code is already present and working correctly.
+
+## Verification
+
+The stream-json reader spawn call is correctly implemented in `src/session.rs` at lines 359-377:
+
+```rust
+// Check if phase changed to PromptInjected and notify watchdog
+let current_phase = startup.phase();
+if last_phase != *current_phase && current_phase.is_prompt_injected() {
+    watchdog_state_clone.mark_prompt_injected();
+
+    // Spawn stream-json reader at PROMPT_INJECTED for stream-json output
+    if matches!(output_format, crate::cli::OutputFormat::StreamJson) {
+        // Calculate byte offset: current transcript file size, or 0 if not exists
+        let start_offset = std::fs::metadata(&transcript_path)
+            .map(|m| m.len())
+            .unwrap_or(0);
+
+        stream_json_handle = Some(emitter::spawn_stream_json_reader(
+            transcript_path.clone(),
+            start_offset,
+        ));
+        stream_json_spawned_clone.store(true, std::sync::atomic::Ordering::SeqCst);
+    }
+}
+```
+
+### Acceptance Criteria - All Met
+
+- ✅ Spawn call is in event loop callback at PROMPT_INJECTED
+- ✅ Only spawns when output_format is StreamJson (line 364)
+- ✅ Passes transcript_path correctly (line 371)
+- ✅ Passes start_offset correctly (lines 366-368, captures current transcript size)
+- ✅ Stores handle in stream_json_handle for later joining (line 370)
+- ✅ Code compiles without errors (verified with `cargo check`)
+
+### Cleanup Paths
+
+The handle is properly joined on all exit paths:
+- Success path (lines 442-446): Sends drain signal, joins
+- Timeout path (lines 404-407): Drops without drain (immediate exit), joins
+- Child exit path (lines 460-463): Drops without drain, joins
+- Interrupt path (lines 469-472): Drops without drain, joins
+
+## Conclusion
+
+This bead's work was already completed in a previous implementation. The code correctly wires the stream-json reader spawn at the PROMPT_INJECTED transition in the session flow's event loop callback.