docs: rename pcbkeepout → keepout (matches actual core element name)

[gsdali]

Summary

The element doc was filed at elements/pcbkeepout.md with the example showing <pcbkeepout shape="rect" .../> — but @tscircuit/core's element catalogue registers this component as Keepout, exposed via JSX as <keepout>. Trying to use <pcbkeepout> in actual circuit code fails the build:

error: Unsupported component type "pcbkeepout". No element with this name
is registered in the @tscircuit/core catalogue.

This is a pure docs mismatch — the runtime element exists and works correctly under the name <keepout> (verified with a 5-line repro that produces a pcb_keepout record in circuit.json).

Changes

• Rename elements/pcbkeepout.md → elements/keepout.md
• Update the example tag <pcbkeepout ...> → <keepout ...> in that file
• Update SKILL.md element catalog: <pcbkeepout /> → <keepout />, repositioned alphabetically (between <jumper> and <led>)
• Add note in the keepout doc pointing at <copperpour outline=...> as the alternative for "exclude pour fill from a region"
• Document the outline prop on <copperpour> with a worked example (antenna keep-out polygon) — same use case, different mechanism

Test plan

• <keepout shape="rect" pcbX={0} pcbY={0} width="6mm" height="4mm" /> builds successfully and produces a pcb_keepout record in circuit.json
• No mention of pcbkeepout remains in the skill repo: grep -r pcbkeepout . returns nothing
• SKILL.md element list is alphabetically sorted

Why

Hit while trying to add an antenna keep-out for an InsightSiP ISP3080-UX module. Followed the doc, used <pcbkeepout>, build crashed. Wasted ~10 minutes diagnosing before grepping register-catalogue.ts to find the actual export name. Renaming the doc + example will save the next person that time.

The PcbKeepoutProps type name in @tscircuit/props is unchanged — only the JSX tag and skill doc are affected.

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