Retire the agent-ok / by-agent label mechanism

[TzuH-Hsu]

Summary

Drop the agent-ok (self-service work queue) and by-agent (audit trail) labels from the template: declaration, labeler automation, issue-form checkbox, and all docs/skills that tell agents or humans to use them.

Context

Neither label has a consumer. agent-ok is set by the issue labeler from a form checkbox and read by nothing; by-agent has neither producer nor consumer and depends on an agent remembering a manual gh pr edit. The "Agent queue" Project view is a hand-built checklist item bootstrap never creates. Real usage is explicit human dispatch, and provenance already lives in commit Co-Authored-By trailers and the PR author account. Recorded as ADR-0007.

Acceptance criteria

• .github/labels.yml no longer declares agent-ok or by-agent
• issue-labeler.yml no longer adds or manages agent-ok; priority/area/type sync unchanged
• Issue forms no longer show an "Agent eligibility" checkbox
• AGENTS.md, PROJECT_FIELDS.md, README, design principles, project-views, and the agent-workflow / pr-authoring / issue-writing / anti-patterns / code-review skills no longer reference either label
• ADR-0007 records the decision and alternatives
• make verify passes
• grep -rn "agent-ok\|by-agent" hits only ADR-0003, ADR-0007, and CHANGELOG history