Add explicit lifecycle modes for unattended watch execution #1479

Parent: #1477

Goal

Make unattended PR lifecycle authority explicit instead of leaving it to agent discretion and ambient credentials.

Scope

Add three lifecycle modes:

• draft-only
• human-gated
• �uto-complete

Use draft-only as the safe default for public-facing and new deployments. Allow trusted autonomous repositories to opt into �uto-complete without introducing interactive Copilot approvals.

Enforcement

• Treat prompt instructions as advisory rather than sufficient enforcement.
• Bind auto-complete to GitHub required checks, branch protection, stable merge state, and an identity that cannot self-approve its own change.
• Verify the resulting PR state and surface policy violations.
• Document the GitHub administrator settings and scoped credentials required for each mode.

Compatibility

• Existing users already producing drafts should see no change.
• Users relying on agents to open ready PRs may need to select a lifecycle mode explicitly.
• Trusted research or internal repositories can retain fully unattended completion by configuring �uto-complete and the required server-side gates.

Acceptance criteria

• Lifecycle mode is typed, validated, and visible in diagnostics.
• Draft-only runs cannot silently become auto-complete runs.
• Auto-complete is rejected when required GitHub protections or independent identity checks are absent.
• Migration guidance and end-to-end tests cover all three modes.