type and tool refactor

[AdminTurnedDevOps]

1. Model conversion: config.Model.ToGuest() / config.ModelFromGuest() is the only host YAML ↔ guest JSON mapping; session config disk, set_model, and the guest agent all use it.
2. History = events: protocol.HistoryLine is an alias of protocol.AgentEvent, so resume, get_context, and live TUI events share one JSON shape.
3. Env-name check: config.ValidEnvName is the single alphabet for MCP credential_env and credentials.Save.
4. Defaults catalog: One provider table plus vmmconfig resource/exec defaults, protocol.GuestRepoDir, and config.GuestImagePath / SecretsFromEnv() replace the scattered 1/768, three API keys, /work/repo, and golden-image strings.
5. Builtin tools: tools.BuiltinSpecs plus CallBuiltin / CallTool is the registry; guest RPC handle and the model execTool path both go through it, so a sixth tool is one spec and one switch.