feat: graceful SIGINT/SIGTERM for soma-daemon

[rmems]

Summary

Handle SIGINT / SIGTERM in soma-daemon so the tick loop shuts down cleanly instead of hard-exiting mid-tick.

Context

Binary: src/bin/soma_daemon.rs. Prefer signal handling that works on Linux CI and local dev; document Windows limits if any.

Implementation plan

1. Install signal handlers (e.g. ctrlc or tokio/std signal) for SIGINT/SIGTERM
2. Set a shutdown flag consumed by the main loop
3. Flush logs / stop cleanly; exit 0 on graceful stop
4. Unit or integration test where practical (or documented manual verification)

Acceptance criteria

• SIGINT/SIGTERM stop the daemon without panic
• Behavior documented briefly in README or REVIEW.md
• Default tests still green
• Assignee: @rmems

Relationships

• Milestone: v0.2 — Stub quality
• Binary-only; no registry redesign

Out of scope

• Kubernetes lifecycle probes
• Multi-process supervision

[linear-code]

LIM-1012