fix(GH-34): persist AgentAPI state

[au-re]

Summary

• Add optional agent.stateFile support and pass it to AgentAPI as --state-file.
• Require native PTY templates with conversationRestore: "supported" to place the state file below a persistence mount.
• Reject AgentAPI state persistence and supported conversation restore for native ACP templates because AgentAPI v0.12.2 does not support that combination.
• Document the fixed 127.0.0.1 AgentAPI service host and the PTY-only state persistence contract.
• Add regression tests and a CLI patch changeset.

Fixes #34

Validation

• bun run check
• bun run typecheck
• bun run test
• bun run build
• bun run pack:check
• bun changeset status
• git diff --check origin/main...HEAD