docs: point the PR-description rules at the template instead of copying it

[0xKT]

Summary

AGENTS.md section 3.7 reproduced .github/pull_request_template.md inline, and
the copy had drifted from it. Every heading differed, and the documented Type
list had no Refactor or CI / tooling -- so a refactor PR following this file
had to pick Others, while the real template offers the right box. The same
section tells you the checkbox mirrors the commit type, which a list missing two
commit types cannot do.

Reference the file rather than restating it: a copy is what drifts. The filling
rules now name the template's actual sections and say what each is for --
Verification wants the commands and their result rather than a claim that they
ran, Related Issues distinguishes Fixes #NNN from a bare reference, and
Type is a mirror of the commit type rather than a fresh judgement call.

Net effect is 37 lines removed and 7 added: the rules that are not in the
template stay, the duplicate of the template goes.

Type

• Fix
• Feature
• Docs
• CI / tooling
• Refactor
• Other

Verification

uv run pre-commit run --files AGENTS.md      # passed
make check-large-files                        # passed
npx commitlint --from origin/main --to HEAD --config commitlint.config.cjs
grep -rn "Change description|Type of change|Checklists" AGENTS.md   # no matches left

CLAUDE.md is a symlink to AGENTS.md, so both read the corrected text.

• Relevant tests pass locally
• Relevant lint / type checks pass locally
• User-facing docs or screenshots are updated when needed

Risk

Documentation only; no code path changes. The advice it replaces was already
unusable as written, so following the new text cannot produce a worse PR than
following the old one. Rollback is a revert.

• Security impact considered
• Backward compatibility considered
• Rollback path is clear for risky changes

Related Issues

Fixes #241