feat: Add --project flag or project picker for multi-project workflows

[ezra-letta]

Problem

Users with multiple Letta Cloud projects cannot easily switch between them in the CLI. OAuth tokens are cached per-session and locked to a single project, causing 404 errors when trying to access agents in other projects.

Current behavior

• letta login authenticates via OAuth but does not offer project selection
• The cached token is tied to one project
• letta --agent <id> returns 404 for agents in other projects
• Project-specific .letta/config.yaml does not override the cached OAuth token

Workarounds

1. Use LETTA_API_KEY env var per project (bypasses OAuth entirely)
2. letta logout + letta login each time (painful for frequent switching)

Proposed solutions (any of these would help)

1. --project <name-or-id> flag - Select project at runtime
2. Project picker during OAuth - Let users choose which project to authenticate to
3. /project interactive command - Switch projects within a session
4. Multi-profile support - Store multiple OAuth tokens keyed by project

User context

From Discord user audrebytes, who has multiple Letta Cloud projects and wants to use different agents across them without re-authenticating each time.

Discord thread: https://discord.com/channels/1161736243340640419/1475340286111776768