feat(heartbeat): skip API calls when HEARTBEAT.md is effectively empty (#1535)

* feat: skip heartbeat API calls when HEARTBEAT.md is effectively empty

- Added isHeartbeatContentEffectivelyEmpty() to detect files with only headers/comments
- Modified runHeartbeatOnce() to check HEARTBEAT.md content before polling the LLM
- Returns early with 'empty-heartbeat-file' reason when no actionable tasks exist
- Preserves existing behavior when file is missing (lets LLM decide)
- Added comprehensive test coverage for empty file detection
- Saves API calls/costs when heartbeat file has no meaningful content

* chore: update HEARTBEAT.md template to be effectively empty by default

Changed instruction text to comment format so new workspaces benefit from
heartbeat optimization immediately. Users still get clear guidance on usage.

* fix: only treat markdown headers (# followed by space) as comments, not #TODO etc

* refactor: simplify regex per code review suggestion

* docs: clarify heartbeat empty file behavior (#1535) (thanks @JustYannicc)

---------

Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/auto-reply/heartbeat.test.ts

@@ -1,6 +1,10 @@
 import { describe, expect, it } from "vitest";
 
-import { DEFAULT_HEARTBEAT_ACK_MAX_CHARS, stripHeartbeatToken } from "./heartbeat.js";
+import {
+  DEFAULT_HEARTBEAT_ACK_MAX_CHARS,
+  isHeartbeatContentEffectivelyEmpty,
+  stripHeartbeatToken,
+} from "./heartbeat.js";
 import { HEARTBEAT_TOKEN } from "./tokens.js";
 
 describe("stripHeartbeatToken", () => {
@@ -105,3 +109,76 @@ describe("stripHeartbeatToken", () => {
     });
   });
 });
+
+describe("isHeartbeatContentEffectivelyEmpty", () => {
+  it("returns false for undefined/null (missing file should not skip)", () => {
+    expect(isHeartbeatContentEffectivelyEmpty(undefined)).toBe(false);
+    expect(isHeartbeatContentEffectivelyEmpty(null)).toBe(false);
+  });
+
+  it("returns true for empty string", () => {
+    expect(isHeartbeatContentEffectivelyEmpty("")).toBe(true);
+  });
+
+  it("returns true for whitespace only", () => {
+    expect(isHeartbeatContentEffectivelyEmpty("   ")).toBe(true);
+    expect(isHeartbeatContentEffectivelyEmpty("\n\n\n")).toBe(true);
+    expect(isHeartbeatContentEffectivelyEmpty("  \n  \n  ")).toBe(true);
+    expect(isHeartbeatContentEffectivelyEmpty("\t\t")).toBe(true);
+  });
+
+  it("returns true for header-only content", () => {
+    expect(isHeartbeatContentEffectivelyEmpty("# HEARTBEAT.md")).toBe(true);
+    expect(isHeartbeatContentEffectivelyEmpty("# HEARTBEAT.md\n")).toBe(true);
+    expect(isHeartbeatContentEffectivelyEmpty("# HEARTBEAT.md\n\n")).toBe(true);
+  });
+
+  it("returns true for comments only", () => {
+    expect(isHeartbeatContentEffectivelyEmpty("# Header\n# Another comment")).toBe(true);
+    expect(isHeartbeatContentEffectivelyEmpty("## Subheader\n### Another")).toBe(true);
+  });
+
+  it("returns true for default template content (header + comment)", () => {
+    const defaultTemplate = `# HEARTBEAT.md
+
+Keep this file empty unless you want a tiny checklist. Keep it small.
+`;
+    // Note: The template has actual text content, so it's NOT effectively empty
+    expect(isHeartbeatContentEffectivelyEmpty(defaultTemplate)).toBe(false);
+  });
+
+  it("returns true for header with only empty lines", () => {
+    expect(isHeartbeatContentEffectivelyEmpty("# HEARTBEAT.md\n\n\n")).toBe(true);
+  });
+
+  it("returns false when actionable content exists", () => {
+    expect(isHeartbeatContentEffectivelyEmpty("- Check email")).toBe(false);
+    expect(isHeartbeatContentEffectivelyEmpty("# HEARTBEAT.md\n- Task 1")).toBe(false);
+    expect(isHeartbeatContentEffectivelyEmpty("Remind me to call mom")).toBe(false);
+  });
+
+  it("returns false for content with tasks after header", () => {
+    const content = `# HEARTBEAT.md
+
+- Task 1
+- Task 2
+`;
+    expect(isHeartbeatContentEffectivelyEmpty(content)).toBe(false);
+  });
+
+  it("returns false for mixed content with non-comment text", () => {
+    const content = `# HEARTBEAT.md
+## Tasks
+Check the server logs
+`;
+    expect(isHeartbeatContentEffectivelyEmpty(content)).toBe(false);
+  });
+
+  it("treats markdown headers as comments (effectively empty)", () => {
+    const content = `# HEARTBEAT.md
+## Section 1
+### Subsection
+`;
+    expect(isHeartbeatContentEffectivelyEmpty(content)).toBe(true);
+  });
+});