fix: reduce cloud sync heartbeat interval from 30s to 60s

[khaliqgant]

Summary

• Increases default daemon heartbeat interval from 30s to 60s, halving cloud API load
• With 11+ active daemons each making 5 API calls per heartbeat cycle (heartbeat, messages, agents, messages/sync, metrics), the cloud server was handling ~22 req/sec on a single 512MB Fly.io instance
• Adds AGENT_RELAY_HEARTBEAT_INTERVAL env var for per-deployment tuning without code changes

Impact

• Cloud API load: ~22 req/sec → ~11 req/sec
• Message/agent sync freshness: 30s → 60s (acceptable for cross-machine discovery)
• Env var allows reverting to 30s or tuning further per workspace

Test plan

• Verify daemon starts and heartbeats at 60s interval
• Verify AGENT_RELAY_HEARTBEAT_INTERVAL=30000 overrides back to 30s
• Monitor cloud API request rate after deploy

🤖 Generated with Claude Code

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[devin-ai-integration]

Devin Review found 1 potential issue.

View 4 additional findings in Devin Review.

[devin-ai-integration]

🟡 Negative or very small env var value bypasses || fallback, causing rapid-fire heartbeats

If AGENT_RELAY_HEARTBEAT_INTERVAL is set to a negative value (e.g., "-5000"), parseInt returns -5000, which is truthy and passes through the || chain without falling back to the 60000 default. When passed to setInterval at cloud-sync.ts:202-204, Node.js clamps values ≤ 0 to 1ms, causing heartbeats to fire ~1000 times/second — the exact opposite of this PR's goal of reducing API load.

Root Cause

The || operator on line 122-124 only guards against falsy values (0, NaN, undefined). Negative numbers are truthy in JavaScript:

heartbeatInterval: config.heartbeatInterval
  || (process.env.AGENT_RELAY_HEARTBEAT_INTERVAL ? parseInt(process.env.AGENT_RELAY_HEARTBEAT_INTERVAL, 10) : 0)
  || 60000,

If the env var is "-1000", parseInt("-1000", 10) returns -1000 (truthy), so the final || 60000 is never reached. Similarly, a very small positive value like "10" (10ms) would also pass through, causing excessive API calls.

Impact: An operator misconfiguring the env var with a negative or very small value would cause the daemon to flood the cloud API with requests, potentially causing a denial of service on the cloud server — which is the very problem this PR aims to mitigate.

Suggested change

	heartbeatInterval: config.heartbeatInterval
	|| (process.env.AGENT_RELAY_HEARTBEAT_INTERVAL ? parseInt(process.env.AGENT_RELAY_HEARTBEAT_INTERVAL, 10) : 0)
	|| 60000, // 60 seconds (reduced from 30s to lower cloud API load)
	heartbeatInterval: config.heartbeatInterval
	|| Math.max(10000, (process.env.AGENT_RELAY_HEARTBEAT_INTERVAL ? parseInt(process.env.AGENT_RELAY_HEARTBEAT_INTERVAL, 10) : 0))
	|| 60000, // 60 seconds (reduced from 30s to lower cloud API load)

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[devin-ai-integration]

Devin Review found 1 new potential issue.

View 6 additional findings in Devin Review.