docs: closing keyword goes in the PR description, once per issue

[0xKT]

Summary

GitHub pairs one closing keyword with one reference. Fixes #NNN, #MMM closes
the first and merely mentions the second. PR #279 wrote seven issues after a
single keyword and left six of them open after merge; they were closed by hand.

Two one-line changes state the rule where a description actually gets written:

• .github/pull_request_template.md - the Related Issues hint.
• AGENTS.md, section 3.7 - the Related Issues filling rule.

A third drops Closes #123 from the commit-footer list in section 3.1. That
line was wrong on its own terms: section 3.3 already records that this repo
squash-merges and drops individual commit bodies, so a closing keyword in a
commit footer never reaches main and closes nothing.

The template hint also stops naming #123, which is a live open issue here, and
a placeholder should not be a real number.

Type

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

Verification

Documentation only; no code paths change.

uv run pre-commit run --from-ref origin/main --to-ref HEAD   -> all hooks Passed or Skipped
make lint-python                                             -> ruff check / format --check clean
make test-python                                             -> 5525 passed, 48 skipped,
                                                                13 deselected in 271.05s
npx commitlint --from origin/main --to HEAD --config commitlint.config.cjs   -> OK
PYTHONPATH=. uv run python scripts/check_commit_messages.py origin/main..HEAD -> OK
PYTHONPATH=. uv run python scripts/check_large_files.py origin/main..HEAD     -> OK

grep -nP "[^\x00-\x7F]" .github/pull_request_template.md     -> 0 matches. The template is
    embedded into every PR description, which CI lints as ASCII-only, so it has to stay ASCII.

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

Risk

No behaviour change. The visible effect is a slightly longer hint under
Related Issues when a new PR opens.

Rollback is a revert of this branch.

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

Related Issues

N/A