docs: drop the local PR template, inheriting the org default

[lesnik512]

Why

This repo's .github/PULL_REQUEST_TEMPLATE.md does not reach the party writing the
PR. gh pr create reads the template only in its interactive editor path or when
passed -T/--template; --body and --body-file skip it, and gh 2.98.0's help
says so outright. Every PR here is opened by an agent through gh, so the file has
never been applied.

The measurement is in modern-python/.github#53. The short version: the convention
checklist appears in 0 of the last 20 merged PRs in faststream-outbox, 0 of 11 in
chat-app, and none in modern-di since #432. Meanwhile the
Why/Design/Non-goals/Verification shape spread widely, including into
modern-python/.github, whose own template said Summary/Changes/Checklist. The
shape tracks AGENTS.md, which is loaded every turn. It does not track the template.

The checklist also duplicated AGENTS.md: its four routes restate the "Where a fact
goes" table and the two-things-outlive-the-PR paragraph. By the admission check, a
fact derivable from a file the agent already loads does not get written.

Design

Delete the template. The repo falls back to the org default, which is a plain
lint/type/test/build checklist true under either convention (modern-python/.github#53
removes its one pre-convention line).

AGENTS.md named the template as the carrier of the body shape. That paragraph is
dropped rather than rewritten: the body shape is expected to come from the agent-skills
convention, so AGENTS.md stops asserting it. What remains in ## Workflow is the
routing that has no other home — what outlives the PR, and where it goes.

Non-goals

• The per-section guidance is dropped, not relocated, and so is the shape
  statement itself. Nothing about PR-body structure moves into AGENTS.md.
• The ADR revisit-trigger clause is not added. It was in some templates and not
  others, and is not being propagated.
• No drift check. With no local template, there is nothing left to silently
  override the default, which was the only reason to keep a local copy.

Verification

Markdown only; no code, config, or CI touched. The deleted file was referenced from
exactly one place, the AGENTS.md sentence updated in the same commit.

Part of modern-python/.github#53.