fix(fleet): complete heartbeat reader/writer consistency + sidecar hardening

F3 follow-on to #595 (HB consistency) — the items flagged in the #595 review:
- defaultMosaicHome() honors MOSAIC_HOME env (not just --mosaic-home flag), so the
  reader matches the writer/launcher when MOSAIC_HOME is set in the shell. The
  systemd guard now checks the LITERAL ~/.config/mosaic (units use %h paths).
- heartbeatPath() honors MOSAIC_HEARTBEAT_RUN_DIR (the writer sidecar's override).
- sidecar: printf %q the interpolated hb path / pid / interval (defense-in-depth).
- vitest: heartbeatPath env-resolution coverage.

Deferred to next F3 milestone (need deeper code work): agent-watch viewer-leak
try/finally fix, and the test-start-agent-session.sh workdir-assumption fix.

Refs #588 #542

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

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

@@ -2892,3 +2892,33 @@ describe('fleet init wizard', () => {
     expect(content).toContain('name: coder0');
   });
 });
+
+describe('fleet ps — heartbeat path resolution', () => {
+  const savedRunDir = process.env.MOSAIC_HEARTBEAT_RUN_DIR;
+  const savedHome = process.env.MOSAIC_HOME;
+  afterEach(() => {
+    if (savedRunDir === undefined) delete process.env.MOSAIC_HEARTBEAT_RUN_DIR;
+    else process.env.MOSAIC_HEARTBEAT_RUN_DIR = savedRunDir;
+    if (savedHome === undefined) delete process.env.MOSAIC_HOME;
+    else process.env.MOSAIC_HOME = savedHome;
+  });
+
+  it('honors MOSAIC_HEARTBEAT_RUN_DIR (matches the writer sidecar override)', () => {
+    process.env.MOSAIC_HEARTBEAT_RUN_DIR = '/run/hb';
+    expect(heartbeatPath('agent-x', '/any/home')).toBe(join('/run/hb', 'agent-x.hb'));
+  });
+
+  it('honors MOSAIC_HOME when no explicit mosaicHome is given', () => {
+    delete process.env.MOSAIC_HEARTBEAT_RUN_DIR;
+    process.env.MOSAIC_HOME = '/custom/mhome';
+    expect(heartbeatPath('agent-y')).toBe(join('/custom/mhome', 'fleet', 'run', 'agent-y.hb'));
+  });
+
+  it('falls back to <mosaicHome>/fleet/run by default', () => {
+    delete process.env.MOSAIC_HEARTBEAT_RUN_DIR;
+    delete process.env.MOSAIC_HOME;
+    expect(heartbeatPath('agent-z', '/home/u/.config/mosaic')).toBe(
+      join('/home/u/.config/mosaic', 'fleet', 'run', 'agent-z.hb'),
+    );
+  });
+});