Support generic ACP session forking

[smsunarto]

Summary

• Enable the existing app, public API, and CLI fork path for generic acp-* providers.
• Parse agentCapabilities.sessionCapabilities.fork during ACP initialization.
• Send unstable ACP session/fork with the source provider session ID, target cwd, and MCP servers.
• Persist the returned child session ID and use it for later turns.
• Reject agents that do not advertise fork support without creating a fresh history-free session.
• Accept the nullable configOptions response defined by ACP SDK 1.3.0.

This is provider-generic and has no acp-amp special case.

Implementation notes

The existing server provisioning path already carries the source provider session ID and persists the provider session returned by the host. This change extends the generic ACP adapter and bridge to use that path.

No host-daemon protocol version bump is required because the server-to-daemon fork command already contains the required fork data.

Validation

• Affected package typechecks passed.
• Host-daemon typecheck and build passed.
• Agent providers: 11 tests passed.
• Agent runtime: 947 tests passed.
• App: 2,706 tests passed.
• Server: 1,499 tests passed.
• ESLint, Prettier, and git diff --check passed.

AGENT GENERATED: by Amp