Persist multi-account X sessions across Chrome restarts (or fail closed if required handle missing)

[Trace824]

Summary

After Chrome restart driven by compose/debug recovery, X AccountSwitcher may omit a previously signed-in handle (or show “Something went wrong. Try reloading.”). list_credentials can be empty, so autonomous re-login is impossible and compose must stop until a human restores sessions.

Repro

• Context: Multiple X accounts previously available in AccountSwitcher.
• Steps:
  1. Restart Chrome (or extension-driven browser restart).
  2. Open AccountSwitcher.
  3. Call list_credentials via MCP.
• Observed: required handle missing from switcher; vault list empty; cannot continue account-sensitive compose safely.

Code pointers (verified against /workspace/FSB)

• mcp/src/tools/vault.ts — list_credentials (~L32+).
• extension/site-guides/social/twitter.js — AccountSwitcher selector.
• extension/ai/tool-definitions.js — credentials tool names in registry lists.
• Chrome profile cookie/session persistence is largely browser/profile ops, not a single FSB API.

Proposed fix

Ops: pin a durable Chrome user-data-dir and avoid wiping cookies. Product: detect missing required handle early (see issue 19) and fail closed rather than posting. Optional: document vault unlock requirements when credentials are expected.

Acceptance / regression

• After restart, either sessions remain or tools error early with “required account missing.”
• No silent compose as a fallback account.

Owner

ops (profile persistence) + extension (fail closed)

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Priority: P2 · Owner: ops