Limit Linear scan cadence

[yvette-carlisle]

Summary

• remove serve --interval and keep the local control-plane tick hardcoded at 15s
• gate Linear-backed queue/status scans behind a fixed 5-minute per-project cadence
• add POST /api/linear-scan so operators/agents can request the next scan explicitly
• update docs and Decodex plugin skills with the new cadence and trigger flow

Verification

• cargo make checks