feat(workflow): gate queued agent phases on four-axis labels

[purp]

Summary

Move queued agent planning and implementation transitions to the four-axis workflow labels.

Related Issue

Closes #3812
Part of #3807
Stacked on #3811

Changes

• Add a read-only phase gate for queued planning and implementation.
• Check acceptance or roadmap placement, phase-specific needs, agent readiness, and an existing plan before implementation.
• Preserve direct human instructions as an override that reports label discrepancies.
• Update build, spike, and specialized security skills to use the new label contract.

Testing

• mise run pre-commit passes
• Unit tests added/updated for phase gating
• E2E tests added/updated (not applicable)
• mise run ci passes with an isolated Go lint cache

Checklist

• Follows Conventional Commits
• Commits are signed and signed off (DCO)
• Architecture docs updated (not applicable; contributor workflow docs updated)

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[purp]

/ok to test 6e24891