docs: the one-sentence rule was written down but nothing enforced it

[skyoo2003]

Summary

#152 wrote the one-sentence changelog rule into RELEASE.md, but the only thing enforcing it was a manual pre-flight checkbox read once per release. changie new now refuses a body over 400 characters, and the rule is stated where a contributor actually lands rather than only behind a link.

Related Issue

Refs #152

Changes

• Enforcement — .changie.yaml sets body.maxLength: 400. changie new rejects a longer body at authoring time, so a paragraph never reaches changes/unreleased/.
• RELEASE.md — the cap is stated beside the rule, with its two limits: it is a ceiling for the two-sentence case rather than a target, and it does not see a hand-written fragment.
• CONTRIBUTING.md and the PR checklist — both said "add a Changie fragment" and linked RELEASE.md without saying what the rule is; both now state "one sentence, two at most" and link the section anchor.
• Unreleased fragments — the docs: the docs restated the same numbers in five different values #151 fragment ran 482 characters, over the new cap, and is shortened to 397 with nothing dropped. The docs: changelog entries told the whole story where one sentence would do #152 fragment covers the cap and the two added surfaces (398 characters).

Files Changed

File	Change
.changie.yaml	Modified — body.maxLength: 400
RELEASE.md	Modified — cap documented in the fragment section and the changie new paragraph
CONTRIBUTING.md	Modified — rule stated, anchor link
.github/pull_request_template.md	Modified — rule stated in the checklist item, anchor link
changes/unreleased/Documentation-20260906-234500.yaml	Modified — #151 body 482 → 397 chars
changes/unreleased/Documentation-20260906-185423.yaml	Modified — #152 body updated to cover this change

Test Plan

• changie new -k Fixed -b <401 chars> -m Issue=1 -d → Error: input length too long: length of 401 > 400; the same call with 398 characters renders the fragment.
• changie batch v1.2.0 --dry-run renders both unreleased fragments unchanged in shape.
• sed -n 's/^ - label: //p' .changie.yaml (the kinds extraction in release.yml:222) still yields all seven kinds — the new body: block does not perturb it.
• Every entry in CHANGELOG.md (78) is still one or two sentences; no changelog output changes in this PR.

Checklist

• Self-reviewed the code
• Added/updated tests — N/A (config and docs; enforcement verified by running changie new, see Test Plan)
• Lint/format passes (golangci-lint run) — no Go files touched; pre-commit hooks passed
• Updated documentation (if applicable)
• Added a Changie changelog fragment for user-facing changes — the existing unreleased docs: changelog entries told the whole story where one sentence would do #152 fragment is extended rather than duplicated