Token harness refresh

[mdeyell-valid-mind]

Adds support to the agent harness integrations so short-lived OAuth access tokens can be refreshed automatically.

Changes

• Added token command execution, expiry parsing, caching, and one-time 401 retry support to:
  • Amp plugin
  • Pi extension
  • Shared agent hook
  • Fake agent script
• Supports token command output as either:
  • a raw bearer token
  • JSON containing access_token, accessToken, or token
  • optional expiry fields via expires_in, expires_at, or expiresAt
• Persists token caches for the Node integrations under each integration state directory.
• Updates docs for auth-mode setup and short-lived token usage.
• Treats executing as a pending state while polling approval decisions.