PR template: reduce to the issues-not-PRs banner; move the maintainers' checklist to AGENTS.md and pr-flow #4960

Problem

.github/pull_request_template.md on v2/main carries three things: an HTML comment restating the "issues, not PRs" policy, the Heads up banner, and the maintainers' PR skeleton (Closes #, Description, Server Details, Motivation, How Has This Been Tested?, Breaking Changes, Types of changes, Checklist, Additional context).

Pull requests are turned off in the repository settings as soon as #4939 lands on main. From then on the template is only ever seen by the maintainers, and its one job is the notice. The comment duplicates the banner, and the skeleton and checklist are maintainer procedure that belongs with the rest of it, in AGENTS.md and the pr-flow skill.

Expected

The template holds only the banner:

Heads up: this repository accepts issues, not pull requests, from anyone but the repository maintainers. Please read CONTRIBUTING.md before continuing. If you're not a maintainer, open an issue (and share the prompt you used, if you've already built the change) rather than this PR. To make a server discoverable, publish it to the MCP Server Registry.

The rules that pointed at the template then point at their new home, so each stays true:

• AGENTS.md: "Every PR answers the checklist in .github/pull_request_template.md" states the checklist itself; the project-structure tree entry and the exceptions that mention "the template checklist" are updated.
• pr-flow: step 5 and step 6 give the PR body's sections instead of pointing at the template's.
• client-smoke: "the PR checklist" references resolve to the AGENTS.md rule.

Once merged, .github/pull_request_template.md on #4939's branch is re-synced from v2/main, so it lands on main byte-identical.