Add Agent Mode session handoff prompt

[baron]

Summary

• add a Handoff action to the Agent Mode chat options menu
• copy exact window, workspace, compose-context, and agent-session routing identities
• direct receiving agents to the existing bind_context and agent_manage.extract_handoff authorities
• emit the correct rpce-cli-debug or rpce-cli command for the running build
• fail closed when the captured menu target becomes stale before clipboard mutation

Validation

• push safety preflight: passed
• strict SwiftFormat/SwiftLint: passed (Conductor 1a9f6671-4f95-4084-a1e7-a24f41ed9938)
• AgentChatTitlebarSafetyTests: 7 passed, 0 failed (Conductor 97c4b7d9-a492-45bc-8497-26b463c2dfbd)
• swift build --product RepoPrompt: passed (Conductor a755bbaf-e685-4aae-a746-3aeeeb072eda)
• live debug build/launch: passed (Conductor c8fedec2-8867-4d90-8ba6-e98cf238dcf4)
• full root PR-ready test lane reached its 3600s timeout without reporting a test failure before termination (Conductor eb4311d8-7157-413e-b1bb-d3486d1bc09c); the affected focused suite and product build were rerun successfully as listed above