Add tests for merge agent live telemetry (TP-164 follow-up)

[HenryLach]

Sage code review of TP-164 recommended adding targeted tests for the merge telemetry pipeline. Currently untested:

1. getActiveSessions() — filters registry agents by role='merger' and non-terminal status
2. Telemetry injection precedence — merge snapshot injects into telemetry map only when no JSONL entry exists
3. Terminal snapshot status — successful merge + cleanExit kill correctly produces 'complete' not 'failed'
4. waveIndex propagation — non-zero waveIndex passed from call site appears in snapshot metadata

These are source-inspection or unit tests, not integration tests.