fix(cli): remove sync --install-cron help text — no handler ever existed (#2795) (#2972)

The top-level `gbrain --help` advertised `sync --install-cron` since the
line was first added, but `src/commands/sync.ts` never parsed or handled
the flag — `gbrain sync --install-cron` silently ran an ordinary one-off
sync instead of installing anything, manufacturing false confidence in
the exact durability layer operators reach for it to secure.

git blame shows the line was introduced once (v0.42.29.0 help-text
scaffold) and never touched again — no design intent to recover.
Implementing it would also compete with autopilot, which already owns
this job: `gbrain autopilot --install` runs a self-maintaining daemon
(sync+extract+embed) on a schedule, including a per-source freshness
check that submits `sync` jobs on its own interval. A second, separate
sync-only cron would be a competing scheduler outside the D10
cycle-lock invariant that already keeps autopilot's own targeted-submit
and full-cycle paths from double-processing.

Removed the misleading line and pointed sync's --watch entry at
`autopilot --install`, mirroring the existing `dream` command's
"See also: autopilot --install (continuous daemon)." pattern one
section below. Added regression coverage to
test/cli-help-discoverability.test.ts asserting the help text no
longer promises install-cron and does point at autopilot.

src/cli.ts

@@ -2289,7 +2289,7 @@ IMPORT/EXPORT
   import <dir> [--no-embed]          Import markdown directory
   sync [--repo <path>] [flags]       Git-to-brain incremental sync
   sync --watch [--interval N]        Continuous sync (loops until stopped)
-  sync --install-cron                Install persistent sync daemon
+                                     See also: autopilot --install (continuous daemon).
   export [--dir ./out/]              Export to markdown
   export --restore-only [--repo <p>] Restore missing supabase-only files
         [--type T] [--slug-prefix S] With optional filters

test/cli-help-discoverability.test.ts

@@ -98,6 +98,33 @@ describe('WARN-6 — main `gbrain --help` lists capture/brainstorm/lsd', () => {
   });
 });
 
+describe('#2795 — `sync --install-cron` help line no longer promises an unbuilt feature', () => {
+  test('main `gbrain --help` does not advertise install-cron', () => {
+    // Pre-fix: `sync --install-cron  Install persistent sync daemon` was
+    // listed in the top-level help with no flag parsing or handler behind
+    // it anywhere in src/commands/sync.ts — `gbrain sync --install-cron`
+    // silently ran an ordinary sync instead of installing anything.
+    const { stdout, status } = runCli(['--help']);
+    expect(status).toBe(0);
+    expect(stdout).not.toContain('install-cron');
+    expect(stdout).not.toContain('Install persistent sync daemon');
+  });
+
+  test('main `gbrain --help` points sync users at the real continuous-daemon command', () => {
+    const { stdout } = runCli(['--help']);
+    // autopilot --install already runs sync+extract+embed on a schedule
+    // (docs/architecture/KEY_FILES.md); point discoverability there instead
+    // of promising a separate sync-only cron installer that never existed.
+    expect(stdout).toMatch(/sync --watch \[--interval N\][^\n]*\n\s*See also: autopilot --install/);
+  });
+
+  test('`gbrain sync --help` never listed install-cron either', () => {
+    const { stdout, status } = runCli(['sync', '--help']);
+    expect(status).toBe(0);
+    expect(stdout).not.toContain('install-cron');
+  });
+});
+
 describe('#1175 — main `gbrain --help` SOURCES block matches the real subcommand set', () => {
   test('archive and its lifecycle siblings are listed', () => {
     const { stdout, status } = runCli(['--help']);