feat: guardrailed handoff envelope + active-handoff lifecycle

[4444J99]

GuardrailedHandoffBrief with constraints_locked, files_locked, work_completed, conventions. Auto-write on MCP handoff, auto-clear on verify pass. PR #10. DONE-285.

[github-actions]

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[4444J99]

Engagement disposition: distilled:conductor/fleet_handoff.py. I verified GuardrailedHandoffBrief exists on main with constraints_locked, files_locked, work_completed, conventions, work_type, and verification_required; the active handoff lifecycle writes, reads, and clears .conductor/active-handoff.md. mcp_server.py writes that active handoff during guardrailed dispatch and clears it after successful cross-verification.