docs(contributing): define self-assignment and priority-gate workflow

[back1ash]

Summary

Document an unassigned, self-selected issue backlog while preserving equal Maintainer authority and the strict P0 to P1 to P2 priority gate.

Scope

• Define the meaning and ordering of priority:p0, priority:p1, and priority:p2.
• Define ready, claimed, blocked, released, and completed issue states.
• Describe self-claiming through assignee, status:in-progress, and an issue comment.
• Describe how dependency completion changes status:blocked to status:ready.
• Clarify that issue ownership is temporary coordination responsibility rather than a Maintainer hierarchy.

Acceptance criteria

• CONTRIBUTING.md documents the priority gate and self-assignment flow.
• GOVERNANCE.md confirms equal Maintainer authority regardless of issue ownership.
• A contributor can identify startable work using labels alone.
• The workflow prevents two contributors from unknowingly starting the same issue.
• Releasing or pausing a claimed issue makes it available without losing useful progress context.

Dependencies

None.

Priority gate and claiming

This P0 issue is ready to be claimed now.

To claim it, assign yourself, replace status:ready with
status:in-progress, and comment with a short implementation plan.

Non-goals

• Assigning permanent module owners or Maintainer levels.
• Changing branch protection or required-review enforcement.
• Introducing sprint dates or individual quotas.