Reserved names: a name someone holds is taken; tests stop signing up reserved words #759

Follow-up to #675 (#662), which is now on main.

• Taken before reserved. Signing up, or founding an org, with a name an account already holds answered username_reserved / name_reserved instead of username_taken / name_taken, so test/app.test.ts's duplicate-signup check failed. Only a new name can be reserved now; a look-alike of an existing name still is. test/reserved-names.test.ts says which answer each case gets.
• Tests that signed up reserved words. test/agents.test.ts ("crew", "bot"), test/contents-api.test.ts ("bot") and test/settings.test.ts ("owner") signed up names that are reserved since #662. They use ordinary names now. All three are outside the gate, which is why the forge's check stayed green.

Verified: app, reserved-names, orgs, agents, contents-api and settings all pass. (test/scale.test.ts also fails on main, on event-visibility assertions unrelated to names; not touched here.)

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