docs: state that a PR title is a Conventional Commit

[zgeoff]

Description

The partial required Conventional Commits for commit messages and, separately, noted that a squash merge makes the PR title the commit subject — but that note sat inside the imperative-mood bullet as its rationale, so the type prefix on a PR title was only ever a two-hop inference.

• Gives the PR-title rule its own bullet, with a contrasting pair rather than a description of the rule.
• Leaves the mood bullet intact, minus the clause that was doing someone else's work.
• Says nothing about scopes: observed practice varies between an issue number, a package name, and none at all, and commitlint requires none.

In zgeoff/vers this cost four agents in a row, and two squash-merged commits reached main with no type prefix.

Testing

• AGENTS.md regenerated from the partial

Context

Nothing validates a PR title in CI — commitlint runs only on the commit-msg hook, which never sees it. Wording is the only guard until a repo adds that check.