New skill: perf, numbers or nothing

[ms-shashank]

Performance is the area where a plausible-sounding change is most likely to do nothing, and where "should be faster" is most likely to be accepted.

Scope

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

• Measure first, and say what was measured. No optimisation without a starting number and the command that produced it.
• No claim without a before and after. "This should be faster" is banned wording. If it cannot be measured, say it cannot be measured.
• Find the actual cost, not the ugly code. The nested loop over four items is not the problem; the query in the loop is.
• State the size the improvement holds at. A change that helps at 10,000 rows and hurts at 10 is a trade, not a win, and the report should say so.
• Refuse micro-optimisation without evidence. If the profile does not show it, do not do it.

Acceptance

• Requires a baseline measurement with the exact command before any change
• Requires a paired after-measurement, and forbids claiming an improvement without one
• Requires stating the input size the result holds at
• Says plainly what to do when something is not measurable, rather than guessing
• 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.