refactor(acp): drop parse-command module, consolidate tool-format helpers

- Remove parse-command.ts in favor of logic consolidated in tool-format.ts
- Rewrite tool-format helpers (toolCallFromPart, toolResultFromPart,
  permissionDisplayInfo, fenceWith) as the single source of truth for
  tool call formatting
- Adapt agent.ts to dev's post-namespace-refactor layout (flat exports
  with `export * as ACP` self-reexport, ProviderID/ModelID branded types,
  Hash.fast, InstallationVersion, ConfigMCP.Info)
- Drop listSessions `unstable_` prefix to match SDK method name
- Guard sendUsageUpdate against missing providerID/modelID

Author: validatedev