fix(setup): replace onboarding state atomically

[tang-vu]

Summary

• write persona, setup-progress, and setup-complete JSON through same-directory temporary files
• flush and fsync each complete payload before atomic replacement
• clean temporary files on both success and failure
• cover replacement failure through the authenticated setup API boundary

Why this matters

The dashboard setup endpoints are the production writers for onboarding state. They opened active JSON files directly with w, truncating the current persona or setup marker before serialization completed. A process interruption, disk error, or failed replacement could therefore leave corrupt state and make the wizard regress to defaults. Atomic promotion preserves the last valid file until the new payload is durable.

Overlap check

Searched open and closed PRs for setup persona atomic write, setup-progress.json atomic, and changes to routers/setup.py. No PR covers onboarding-state durability. Existing atomic-write work targets .env, OAuth state, and extension receipts, not setup state.

Test plan

• python -m pytest tests/test_setup.py tests/test_routers.py -q — 79 passed
• python -m py_compile routers/setup.py tests/test_setup.py
• git diff --check

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