Make it yours: copies start private, and an account makes at most five a day #886

Closes #852 Closes #853

Two follow-ups to Make it yours (#606, #835), one pull.

#852: private by default. The plan (page, API, MCP) now says who will see the copy. On the page it is a choice: "Only you, until you open it up" (the default) or "Anyone, from the start", the second offered only when the original is public. The API and MCP take visibility (private by default). A copy of a private original is never public, whoever asks; the request is refused as invalid. The copy and its app take the choice before the app is registered, so nothing of the copy is public by accident.

#853: at most five copies a day. An account may make at most COPIES_PER_DAY (5) copies in any 24 hours. Each copy is a repository plus an app that deploys at once. The plan shows how many are left ("N more copies you can make in the next day"). Past the limit the answer is a refusal with when another may be made: on the page a flash, on the API a 429 with resetsAt, on MCP limit. Nothing is made. Landing on a copy already made does not count. The constant is the setting for now; making it admin-settable is for later.

Tests. test/make-it-yours.test.ts has two new tests:

• a default copy is private, code and app; an explicit public copy of a public original is public; a public copy of a private original is refused, even for a site admin;
• five copies are made; the sixth is refused by the action (limit, resetsAt in the future) and the API (429), and none is made; a day later the count resets; landing on an existing copy still works.

Local runs: make-it-yours, explore, mcp, studio and the cross files give 290 pass. tsc is clean.

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