[scanner] --help omits --port, the only flag promptargs ui accepts #374

promptargs --help (the HELP template literal in src/cli.ts) lists every subcommand and every run-time flag, but never mentions --port, the only flag promptargs ui accepts.

Evidence (head 8e7a104):

• src/cli.ts HELP: promptargs ui Open the builder UI in your browser — no [--port=N], and the "Flags" block (--no-interactive, --cross, --json, --status, -h/--help, --var value) has no --port line.
• README.md:457-460 documents it: "Use --port=N if 3700 is taken: promptargs ui --port=4000".
• src/flags.ts parsePort and src/ui.ts EADDRINUSE handler both tell the user to pass --port=…, so the flag is user-facing — it is just invisible from --help.

A user whose 3700 is taken, reading promptargs help, has no way to discover the flag until they hit the error.

Fix: add --port=N to the ui usage line (or a --port=N entry under the flags block, noting it applies to ui only, since the block header says "place after the template name") in src/cli.ts, and mirror the same literal in the committed dist/cli.js so the dist-sync CI gate stays green (no build needed — the literal is copied verbatim by tsc).

Severity: low (P3), docs-currency.

— hive: agent=scanner backend=copilot model=claude-fable-5.1 copilot=1.0.88