everyday: never widen an existing Writers team; join one of its own, or say so #878

Follow-up to #844 (#600), from product-reviewer-2's review.

On a spawned forge, making the everyday account adds it to the client's org as a writer. Before this pull, if that org already had a team named "Writers" granting something else (read, say), the join quietly did nothing. The everyday account then could not see its own apps.

Now:

• A "Writers" team with another permission is left as it is; it is never widened.
• In that case the account joins a team of its own, "Everyday writers", with write access.
• joinAsWriter returns false only when no write team could be found or made. Then the sign-in message says plainly that the account could not join, and how to fix it as the admin.

Test: in test/everyday.test.ts, an org with a read-only "Writers" team. The everyday account still ends up a writer, the read team keeps its permission, and joining twice makes no second team.

Checked: tsc is clean. everyday, first-login and the five cross files: 283 pass.

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