feat(templates): add heartbeat templates with emptiness detection and catchup

scripts/templates/heartbeat/HEARTBEAT.md

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+# Heartbeat: {{AGENT_NAME}}
+
+Read this file on each heartbeat. Follow it strictly. Do not infer or
+repeat old tasks from prior chats. If nothing needs attention, reply
+HEARTBEAT_OK.
+
+## Tasks
+
+tasks:
+  - name: {{TASK_1_NAME}}
+    interval: {{TASK_1_INTERVAL}}
+    prompt: "{{TASK_1_PROMPT}}"
+  - name: {{TASK_2_NAME}}
+    interval: {{TASK_2_INTERVAL}}
+    prompt: "{{TASK_2_PROMPT}}"
+
+## Context
+
+{{CONTEXT_NOTES}}
+
+## Rules
+
+- Only perform tasks listed above
+- Respect the interval — do not run a task before its next due time
+- If a task fails, log the error and continue to the next task
+- Respond with HEARTBEAT_OK if no tasks are due