feat(cli): interactive picker for configure when multiple mailboxes are configured across providers

[stevenobiajulu]

Summary

When more than one mailbox or provider is already configured, email-agent-mcp configure should present an interactive picker rather than requiring the user to remember exact provider names and mailbox aliases on the command line.

Current behavior

Today, the CLI requires the user to know:

1. Which provider flag to pass (--provider microsoft or --provider gmail)
2. Which --mailbox alias they previously used (defaults to default)

If the user runs npx email-agent-mcp configure with no flags, it silently defaults to --provider microsoft (see runConfigure in packages/email-mcp/src/cli.ts:857). For users who set up Gmail months ago, this means typing the wrong command and reauthenticating the wrong account, or hitting trial-and-error until the right combination works.

Desired behavior

If configure is run interactively (TTY) and there is more than one mailbox already configured, present a picker that lists them by:

• email address (the human-meaningful identifier)
• provider (microsoft / gmail)
• mailbox alias (the existing CLI key)
• last-authenticated timestamp (so the user can see which one is stale)

Example (illustrative — exact UX is up to the implementer):

? Which mailbox do you want to (re)authenticate?
  1) steven@usejunior.com           microsoft  alias: work     last auth: 2026-01-12
  2) steven.obiajulu@gmail.com      gmail      alias: personal last auth: 2025-11-03
  3) [add a new mailbox]

Selecting an existing mailbox should run the reauth flow for that exact provider + alias combination. Selecting "add new" should fall through to the existing wizard flow.

Notes

• The smart-default wizard in cli.ts already runs a runWizardMenu when there are existing mailboxes and the user runs npx email-agent-mcp with no command. That wizard could be extended to handle the configure command too, or the picker logic could be lifted into a shared helper.
• For non-TTY callers (CI, MCP host startup), keep the existing flag-driven behavior — the picker should only activate in TTY mode.
• This is a small change but it removes a meaningful papercut for users who don't touch the CLI weekly.

Why this matters

Users who reauthenticate weeks or months after initial setup have to remember exact provider and alias names to pick the right mailbox. A picker removes that memorization burden and eliminates a class of "I reauthed the wrong account" mistakes.

[stevenobiajulu]

Two additional notes from review:

1. Flag precedence: The picker should only activate when the user's intent is ambiguous (no --provider or --mailbox flag passed). If the user explicitly passes --provider gmail or --mailbox personal, the CLI should skip the picker and use the provided flags directly. This maintains compatibility for scripts and power users.

2. Existing infrastructure: listConfiguredMailboxSummaries() already exists in cli.ts and returns provider, alias, email address, and last-auth timestamp — exactly the data the picker needs. No new data-fetching code required.