chore: remove GitHub issue triage workflow

[ThomasK33]

Summary

• Remove the experimental GitHub issue triage automation from the project.
• Delete the project github-issue-triage workflow plus its GitHub action helpers and shared workflow action library.
• Remove the now-unused triage automation/risk label guidance from agent docs.

Behavior changes

• /github-issue-triage is no longer available as a project Mux workflow.
• The repository no longer includes the workflow-owned GitHub issue listing/comment/label helper actions.
• The docs now describe the normal gh issue-tracker conventions only, without triage-workflow setup requirements.

Validation

• mise run format
• npm run format:check
• npm run lint
• npm run typecheck
• mise run test-unit

Dogfooding / proof

• No rendered-output, CLI runtime, or GUI behavior changed; this removes an experimental workflow and docs references, so no screenshot/video proof bundle was generated.

---

Generated with mux • Model: openai:gpt-5.5 • Thinking: xhigh