feat(cli): four documented scripts have no --help — add the usage block (one script per PR)

Time: ~20 min · Difficulty: Beginner

Several user-facing scripts documented in docs/SCRIPTS.md still have no --help, so running one with a wrong flag gives you no way to discover the right one. We already fixed this for stats.mjs, followup-cadence.mjs, detect-reposts.mjs and check-liveness.mjs; these four are the ones left:

• build-dashboard.mjs
• cv-sync-check.mjs
• normalize-statuses.mjs
• plugin-install.mjs

What to do

1. Pick ONE script from the list above.
2. Add a USAGE string and a KNOWN_FLAGS array, and print USAGE and exit 0 when --help or -h is passed.
3. Unknown flags should exit non-zero with the usage block, rather than being silently ignored.
4. Document the real flags the script already accepts: read the argument handling first, do not invent options.

Acceptance

• node <script>.mjs --help prints the usage block and exits 0.
• node <script>.mjs --nonsense prints the usage block and exits non-zero.
• No behaviour change when the script runs normally.
• One script per PR: that keeps review easy and lets four different people take this issue at the same time.

Pattern to mirror

stats.mjs (its KNOWN_FLAGS / USAGE block near the bottom): that is exactly the shape to copy.