docs: keep the generic PR template as a deliberate org default

[lesnik512]

Why

The org default PR template is inherited by every repo without a local one, and its
checklist carries Docs updated if behavior or public API changed. That is the
pre-convention question, the one this org stopped asking: it prompts for a page
update rather than the admission check, and it is untrue for any repo where behaviour
detail lives in INVARIANT: tests rather than prose.

Separately, this file opens by claiming its conventions apply across all repos in the
org. They do not. AGENTS.md is not a GitHub default community health file, so it is
never inherited, and no other repo's AGENTS.md references this one. An agent reads
exactly the file in the checkout it is standing in.

Design

Keep the template as the org default and delete the one untrue line. What remains
(lint, type check, tests, build, metadata) holds under either convention, so the
inherited form is correct everywhere for the first time.

AGENTS.md loses three lines and gains none. The opening claim goes, and nothing
replaces it: an agent reading this file is already standing in this repo and cannot
act on a statement about repos it will never see. The heading follows, becoming
# modern-python/.github to match CONTEXT.md, since "org conventions" asserts the
same reach the paragraph just gave up.

The Workflow blockquote goes for the same reason. It existed to explain why the
template contradicts the convention, to an agent that opens a PR with
gh pr create --body and never reads the template. Its one actionable clause, "write
PR bodies in the shape above", is the paragraph directly above it.

Non-goals

• The local templates in modern-di, faststream-outbox and chat-app. Deleted
  in their own PRs; this one only makes the default they fall back to correct.
• that-depends. Keeps its local template, which is the only one in the org
  measurably in use.
• The ADR revisit-trigger phrase, still on line 91. Delete the local PR templates; keep the generic org default #53 decided not to propagate
  it to the other repos; removing it here would change this repo's own practice and is
  a separate call.
• CONTEXT.md:8, which repeats the org-wide claim just deleted here. It needs the
  same fix and is mid-edit on another branch.

Verification

uv run pytest: 123 passed, 26 skipped, 5 failed. All five failures are PNG
rendering (test_lockups, test_pngopt, test_projects) and reproduce unchanged on
main — rsvg-convert and pngquant are not installed locally. CI installs them
(#62). This change touches no Python.

Part of #53.