chore: add the writing-changelog skill and route CHANGELOG.md edits through it

[V3RON]

Closes: no issue; follows up the agentic setup in #85.

What changed

Changelog entries were leaking implementation detail: the two entries written after #85 landed ran seven to ten lines and named internal functions, processes and a file descriptor. No skill covered the changelog. implement-issue called the changelog line "separate" from the docs written with writing-user-docs, its finish step named no skill, and writing-user-docs has no length rule (its "concise means no filler, not short" pushes the other way) and no example of an entry.

This adds .claude/skills/writing-changelog: one bullet per change, a bold <Kind>: <what the user notices> part plus at most one more sentence, a list of what never goes in an entry, a leaked-versus-clean example pair, and the Breaking kind that cut-release keys the version bump off. Voice and scope rules stay in writing-user-docs and apply by reference.

Wiring so agents reach it without relying on description matching: AGENTS.md rule 6 and the skills table, both changelog steps in implement-issue, a dedicated finding in review-pr, the PR template checklist, the CHANGELOG.md header, and a LESSONS.md section.

Acceptance criteria

Agent setup only; no code, no tests.

#	Criterion	Test	Tier
1	Every skill that touches CHANGELOG.md names writing-changelog	grep -rn writing-changelog .claude/skills AGENTS.md .github	manual

E2E evidence

not applicable

Checklist

• CHANGELOG.md has an entry under Unreleased (writing-changelog skill), or the change is not user-visible
• User-facing docs updated for every surface the change touches (writing-user-docs skill), or the change is not user-visible
• No new import past a module's index.ts; no new direct node:* I/O outside an adapter
• Simplification checklist from the architecture skill applied, exceptions explained above
• docs/ARCHITECTURE.md updated if a surface it describes changed

Out of scope

The existing entries under 0.11.0 and 0.11.1 are not rewritten; released sections stay as published. A PreToolUse hook on CHANGELOG.md edits was considered and left out until the skill references prove insufficient.

Status

Implement: done Review: pending E2E: not applicable Ready: yes