[Task 02] Multi-account skeleton

[lubobill1990]

Task 02 — Multi-account skeleton

Depends on: 01
Unblocks: 03, 04, 12

Goal

Replace the single-token state with an account pool. No service code uses
the new pool yet — that comes in task 03. This step delivers the
data structures, the loader, the picker, and the acquire/release API.

Scope

• New file src/lib/account-pool.ts:
  • Account interface with all fields from design doc 01.
  • AccountPool class with acquire(), release(account), pickByStrategy(),
    markCooldown(account, ms), markFailure(account).
• src/lib/state.ts becomes:

  export interface State {
    accounts: Account[]      // owned by AccountPool but referenced here
    pool: AccountPool
    accountType: string      // default for anonymous accounts
    strategy: 'round-robin' | 'least-busy' | 'least-recent'
    models?: ModelsResponse
    vsCodeVersion?: string
    manualApprove: boolean
    rateLimitWait: boolean
    showToken: boolean
    rateLimitSeconds?: number
  }

  Remove githubToken, copilotToken, lastRequestTimestamp (the last one
  moves onto Account).
• src/lib/token.ts:
  • setupCopilotTokenFor(account) instead of the global helper.
  • Each account starts its own refresh setInterval.
• New file src/lib/accounts-loader.ts:
  • Load accounts.json from --accounts-file.
  • Merge legacy single token (--github-token or github_token file) as
    default if accounts.json absent.
  • Seed accounts table on startup.
• src/start.ts:
  • Add flags --accounts-file, --strategy.
  • Call loadAccounts() then pool.initAll() (each account fetches its
    Copilot token in parallel via Promise.all).
  • Keep the current "we have a token" log line but per account.

Picker rules

pick():
  candidates = accounts.filter(a =>
    a.copilotToken && (a.cooldownUntil ?? 0) <= now
  )
  if candidates.empty: throw "no usable account"

  switch (strategy):
    'round-robin':  return candidates[(cursor++) % candidates.length]
    'least-busy':   return min(candidates, by inFlight, then by lastUsedAt asc)
    'least-recent': return min(candidates, by lastUsedAt asc)

Definition of Done

• Booting with no accounts.json still works (legacy single token).
• Booting with two-account accounts.json logs two refresh loops.
• Each account has its own copilotToken after startup.
• Unit test: AccountPool picker returns least-busy when one account is
  artificially in-flight.
• Unit test: cooldown account is excluded; on cooldown expiry it returns.

---

• Source: docs/tasks/02-multi-account-skeleton.md
• Design: docs/design/
• Phase: foundation
• Depends on tasks: 1