feat: implement acp_export_session tool #28

Summary

Implement the acp_export_session MCP tool to export session configuration and transcript for archival.

Blocked On

• ambient-code/platform PR #625 (frontend export chat feature)

This tool should consume the backend endpoint added by platform PR #625 rather than reimplementing export logic.

API Spec

See issue #27 for the preserved API specification.

Split from #27.