docs(skills): add lean-mcp-tools and changelog-entry

[lukecotter]

Two skills, written from what #87 and #86 taught, so the next change to a tool definition or a changelog entry does not have to relearn it. Contributor tooling only: no source file changes, no changelog entry, nothing shipped in the package.

• lean-mcp-tools — where a fact belongs (a definition costs tokens on every request, instructions once per session, a response once per call), what a definition may hold, restructuring instead of deleting, the z.partialRecord win, measuring the whole wire object, and the three assertions that gate it.
• changelog-entry — the shape of one entry, what earns one, versions and migration notes, and why a stacked-branch conflict in ## [Unreleased] keeps both sides.

Each is one SKILL.md under .claude/skills/, symlinked from ~/.claude/skills/ so it is available in every project.

Tested, not asserted

Both were written test-first, following superpowers:writing-skills. A subagent got the task with no mention of skills, twice: once before the skill existed, once after.

lean-mcp-tools — the baseline invented an annotations: { priority: 6 } hint, dropped readOnlyHint and openWorldHint, restated "in milliseconds" in three per-property descriptions, added an outputSchema, and listed six pre-ship checks, none of which measured anything. With the skill: no invented hint, no duplicated title, "in milliseconds" moved to instructions, outputSchema declined for the double-send reason, and the checks became the per-tool budget, the total against the last published figure, and the keyword assertion — plus a unit test to pin the annotations.

changelog-entry — the baseline wrote five past-tense entries for one user-visible change, promoted z.partialRecord and the spec reasoning into user-facing lines, referenced the issue on the first entry only, and used an inline URL. With the skill: one present-tense line, a reference definition under <!-- Unreleased -->, and the mechanism, the spec detail and the contributor-facing eval check all left out as unobservable to someone upgrading.

Both skills were selected from their description alone, with no prompting.