check-ai-co-authors.sh misses Amp trailers (amp@ampcode.com): 12 ECS-branch commits pass silently

`.github/scripts/check-ai-co-authors.sh` does not catch Amp trailers. 12 commits on `feature/ecs-migration` (`a2603c59a6..907ca2b147`) carry `Co-authored-by: Amp ` and the script prints `No AI agent Co-authored-by trailers found.` and exits 0.

The pattern list's comment says "Anthropic — Claude Code / Amp", but the only Anthropic pattern is `noreply@anthropic\.com`. Amp signs as `amp@ampcode.com`. The generic name catch-alls cover Claude, Copilot, Cursor, Codex, Gemini, Aider, Devin, Windsurf, Cline, Amazon Q, Jules and OpenCode, but not Amp.

Verified 2026-08-23 on `origin/main` with both arms:

- Real range: `bash check-ai-co-authors.sh a2603c59a6 907ca2b147` prints "No AI agent Co-authored-by trailers found.", exit 0, while `git log` counts 12 `amp@ampcode.com` trailers in that range.
- Positive control: a range containing `Co-Authored-By: Claude Opus 5 ` (commit `caaa839b12`) fires correctly, exit 1.

Fix is 2 lines in `AGENT_PATTERNS`:

```bash
# Amp (Sourcegraph)
'amp@ampcode\.com'
'Co-authored-by:.*\bAmp\b'
```

`\bAmp\b` will not match "Amplitude" (word boundary requires a non-word character after "Amp").

Note this gap is why #14246's commits currently pass the check.