Health check: required handles present in AccountSwitcher before compose

[Trace824]

Summary

Compose automation should verify that required X handles appear in AccountSwitcher before focusing the draft. Missing handles after restart currently surface only mid-flow or after failed switch attempts.

Repro

• Context: AccountSwitcher missing one of the expected accounts.
• Steps:
  1. Open [data-testid=SideNav_AccountSwitcher_Button].
  2. Enumerate visible account cells.
  3. Proceed to compose without checking.
• Observed: switch/login paths fail late; list_credentials may not help if vault empty.

Code pointers (verified against /workspace/FSB)

• extension/site-guides/social/twitter.js — accountMenu: '[data-testid="SideNav_AccountSwitcher_Button"]'.
• Likely area: site-guide helper or content execute_js recipe; vault mcp/src/tools/vault.ts for credential metadata only.

Proposed fix

Add a preflight that opens AccountSwitcher (or reads known DOM) and asserts configured handles are listed; fail closed if not.

Acceptance / regression

• Missing handle → clear preflight error; present handles → compose allowed.
• No dependency on unpublished internal APIs.

Owner

extension

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Priority: P2 · Owner: extension