ci: let a squashed Dependabot merge pass on main

[vyncint]

What & why

Merging a Dependabot pull request turns main red, even though the pull
request itself is green. simt-diff hit this at
52cc142
minutes after the carve-out landed.

GitHub composes the squash-merge message itself and appends a co-author
trailer naming the pull request's author. For a Dependabot pull request that
trailer names dependabot[bot], at
49699333+dependabot[bot]@users.noreply.github.com — and bot is in the
vendor list the message rules match against, so the rule fires.

The identity carve-out did not help: this is the message half, which was left
strict on purpose. The result is the worst shape available — the commit passes
on the branch, fails on main, and main is linear and non-fast-forward, so
it cannot be repaired. Exactly the failure check-dco.sh already carries a
composed-squash exemption for.

A co-author trailer naming an allowlisted dependency bot is now dropped before
the message rules are matched. Every other such trailer is matched exactly as
before, and the rest of the message — watermarks, robot emoji, any other
vendor — is still matched in full, including on the bot's own commits.

Tested

tools/test-policy-scripts.sh gains three cases. Run against the current
script they reproduce the bug; against this one, 17/17:

• a GitHub-composed squash naming dependabot as co-author — passes (the fix)
• the same shape naming an AI vendor — still fails
• the same shape naming an unallowlisted other[bot] — still fails

Also run against the real commit that turned simt-diff's main red: rejected
by the current script, accepted by this one.

What this does not do

• No CHANGELOG.md entry: not user-facing.
• Does not repair simt-diff's existing red commit on main; that run stays
  red in history and goes green on the next push.

Checklist

• No issue linked — follow-up to the Dependabot carve-out
• Tested, and the tests can fail — they fail against the current script
• No Rust changed
• All commits are signed off
• No AI attribution trailers