docs: close round-5 token-exchange follow-ups

- settings.toml.example: drop the stale `enable_token_exchange = false`
  deprecated-alias line (the key was removed from config.py _DEFAULTS).
- configuration-migration-v2.md: add a Quick Reference row + note that
  `ENABLE_TOKEN_EXCHANGE` was removed and is now ignored; use
  `MCP_DEPLOYMENT_MODE=login_flow` instead.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

settings.toml.example

@@ -48,7 +48,6 @@ enable_background_operations = false
 # Deprecated aliases (declared so env var overrides work)
 vector_sync_enabled = false
 enable_offline_access = false
-enable_token_exchange = false
 
 # --- Token storage ---
 token_encryption_key = "@none"