feat: matrix launch verification — detect and report failed session starts

[c-h-]

Problem

When agentctl launch --matrix starts multiple sessions, some may fail silently (provider down, model not found, OpenCode hang). The launch command returns success because the wrapper shell starts, but the actual coding agent never runs. Nobody notices.

Proposed Solution

After matrix launch, agentctl should:

1. Wait for each session to produce first output (or configurable timeout, default 60s)
2. Report which sessions are alive vs dead/stalled
3. Return non-zero exit code if any session failed to start
4. Output structured JSON with session status for programmatic consumption

This lets SOP agents check the launch result and relaunch failed sessions immediately, rather than discovering the failure hours later.

Related

• Matrix CWD doesn't expand tilde (~) paths #150, Matrix-launched opencode sessions not registered in daemon state #151, OpenCode exits with code 0 on ProviderModelNotFoundError #152 — matrix launch bugs (all fixed)
• Bug: Lock error message shows 'by ms: undefined' #58 (ohm) — circuit breaker stale state caused provider failures that triggered this