fix(fleet): heartbeat consistency — honor MOSAIC_HOME + MOSAIC_HEARTBEAT_INTERVAL

F3 milestone 1 — the two pre-existing HB bugs (from #584) flagged in the #590 review:
- writer (start-agent-session.sh): MOSAIC_HEARTBEAT_RUN_DIR now defaults to
  ${MOSAIC_HOME:-$HOME/.config/mosaic}/fleet/run, matching the path `fleet ps`
  reads (heartbeatPath uses the resolved mosaicHome). Fixes false "stale/unknown"
  HB on custom MOSAIC_HOME deployments.
- reader (fleet.ts): new heartbeatIntervalMs() honors MOSAIC_HEARTBEAT_INTERVAL
  (seconds); parseHeartbeat's freshness threshold now matches the writer's actual
  cadence instead of a hardcoded 15s. + vitest coverage.

Validated: custom-home writer dir == reader path; interval 30 -> 30000ms, unset
-> 15000ms; prettier clean.

Refs #542 #588

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/mosaic/framework/tools/fleet/start-agent-session.sh

@@ -6,7 +6,7 @@ MOSAIC_TMUX_SOCKET=${MOSAIC_TMUX_SOCKET:-mosaic-factory}
 MOSAIC_AGENT_RUNTIME=${MOSAIC_AGENT_RUNTIME:-pi}
 MOSAIC_AGENT_WORKDIR=${MOSAIC_AGENT_WORKDIR:-$HOME}
 MOSAIC_AGENT_COMMAND=${MOSAIC_AGENT_COMMAND:-}
-MOSAIC_HEARTBEAT_RUN_DIR=${MOSAIC_HEARTBEAT_RUN_DIR:-$HOME/.config/mosaic/fleet/run}
+MOSAIC_HEARTBEAT_RUN_DIR=${MOSAIC_HEARTBEAT_RUN_DIR:-${MOSAIC_HOME:-$HOME/.config/mosaic}/fleet/run}
 MOSAIC_HEARTBEAT_INTERVAL=${MOSAIC_HEARTBEAT_INTERVAL:-15}
 
 if [ -z "$AGENT_NAME" ]; then

packages/mosaic/src/commands/fleet.spec.ts

@@ -851,6 +851,23 @@ describe('fleet ps — heartbeat parsing', () => {
     expect(hb.health).toBe('unknown');
     expect(hb.ts).toBeNull();
   });
+
+  it('honors MOSAIC_HEARTBEAT_INTERVAL for the freshness threshold', () => {
+    const prev = process.env.MOSAIC_HEARTBEAT_INTERVAL;
+    try {
+      // A 60s-old beat is STALE at the default 15s interval (3x15 = 45s)...
+      const ts = new Date(NOW - 60_000).toISOString();
+      const content = `ts=${ts}\npid=1\nstatus=ok\n`;
+      delete process.env.MOSAIC_HEARTBEAT_INTERVAL;
+      expect(parseHeartbeat(content, NOW).health).toBe('stale');
+      // ...but HEALTHY when the operator widened the interval to 30s (3x30 = 90s).
+      process.env.MOSAIC_HEARTBEAT_INTERVAL = '30';
+      expect(parseHeartbeat(content, NOW).health).toBe('healthy');
+    } finally {
+      if (prev === undefined) delete process.env.MOSAIC_HEARTBEAT_INTERVAL;
+      else process.env.MOSAIC_HEARTBEAT_INTERVAL = prev;
+    }
+  });
 });
 
 describe('fleet ps — systemd show parsing', () => {

packages/mosaic/src/commands/fleet.ts

@@ -368,6 +368,16 @@ export function buildAgentTailCommand(
 // ---------------------------------------------------------------------------
 
 export const HEARTBEAT_INTERVAL_MS = 15_000;
+
+/**
+ * Heartbeat interval in ms, honoring MOSAIC_HEARTBEAT_INTERVAL (seconds) so the
+ * `fleet ps` freshness threshold matches the writer sidecar's actual cadence
+ * (start-agent-session.sh). Falls back to HEARTBEAT_INTERVAL_MS (15s).
+ */
+export function heartbeatIntervalMs(): number {
+  const sec = Number.parseInt(process.env.MOSAIC_HEARTBEAT_INTERVAL ?? '', 10);
+  return Number.isFinite(sec) && sec > 0 ? sec * 1000 : HEARTBEAT_INTERVAL_MS;
+}
 export const HEARTBEAT_HEALTHY_MULTIPLIER = 3;
 
 export interface HeartbeatInfo {
@@ -496,7 +506,7 @@ export function parseHeartbeat(content: string | null, nowMs = Date.now()): Hear
       status = val;
     }
   }
-  const thresholdMs = HEARTBEAT_INTERVAL_MS * HEARTBEAT_HEALTHY_MULTIPLIER;
+  const thresholdMs = heartbeatIntervalMs() * HEARTBEAT_HEALTHY_MULTIPLIER;
   let health: 'healthy' | 'stale' | 'unknown' = 'unknown';
   let ageMs: number | null = null;
   if (ts !== null) {