New skill: refactor, behaviour-preserving and proven rather than asserted

[ms-shashank]

A refactor is the one change where "it still works" is the entire acceptance criterion, and it is the change most likely to be claimed rather than proven.

Scope

plugins/refactor/skills/refactor/SKILL.md, tools: full.

• Establish the baseline FIRST. Run the suite before touching anything and record the result. A refactor with no green baseline is not a refactor, it is a rewrite with optimism.
• One behaviour-preserving change at a time, each with the suite run after it. Bundling is how a refactor smuggles in a bug nobody can find later.
• Refuse to bundle a behaviour change. If a fix is spotted along the way, name it and leave it. "I noticed X and did not change it" is the correct output.
• Say what the tests do NOT cover. If a module has no tests, a refactor of it is unproven, and the honest report says so instead of "all tests pass".
• Report the diff by intent, not by file: what moved, what was renamed, what was extracted, and what stayed identical.

Acceptance

• Requires a recorded green baseline before the first edit
• Requires the suite between changes, not only at the end
• Explicitly forbids folding a fix into a refactor, with the wording to use instead
• Requires stating the coverage gap when the touched code is untested
• plugin.json and marketplace.json updated

[ms-shashank]

Assigning this to you, behind #3. Write SKILL_STYLE.md first and then this one against it, so the guide gets tested by the next thing that has to follow it.

I picked this one and #8 for you rather than the others because both are the same shape as the two you have already written: the value is in refusing to claim something without evidence. That is the house style and you have demonstrated it twice.

Same bar as your last two on the PR description: what task you tried it on, and what the answer looked like with and without the skill. And check the tools value against what the skill's own instructions require, which is the thing that came back on #11.