fix: scope promotion revocation guards

[BeforeLights]

Follow-up to the existing promotion review on PR #14.\n\n- scope revocation mutexes by account/workspace so unrelated syncs do not block sign-out\n- add explicit sign-in/reactivation after device-key initialization\n- cover sign-in/sign-out recovery and cross-scope concurrency\n\nValidation: Android unit/compile and connected instrumentation (3/3), corepack pnpm repo:check, repo:build, infra:check, targeted static tests, and git diff --check.\n\nThis is a narrowly scoped promotion-gate fix; it is intentionally not invoking CodeRabbit on the dev PR.

[coderabbitai]

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