Wire issue triage workflow to reusable skill

[PureWeen]

Summary

This is layer 3 of native stack #68872 and depends on #68870.

• declares .github/skills/issue-triage as a compiler-managed local skill in the production workflow
• invokes the skill for reusable area, type, subtype, regression, duplicate, abstention, and semantic-summary decisions
• removes the duplicated inline area/type policy so references/areas.md is the single area-policy source
• carries forward current-main concurrency, explicit target parameters, no-op behavior, deterministic trusted current-type retrieval, and gh-aw v0.87.10 structure
• preserves any non-empty existing type and mechanically stages set-issue-type; only untyped issues can receive exactly one of Bug, Feature, or Task
• stages label/type/removal mutations during dry runs while leaving the dry-run analysis comment active
• preserves workflow-owned activation guards, untrusted-input handling, vulnerability suppression, allowlists, exact mutation ordering, and compiler-managed skill installation

Validation

• full-repository gh aw compile --strict --no-check-update: all 5 workflows compiled with 0 warnings
• focused gh-aw v0.87.10 recompile produced zero diff
• generated safe_outputs explicitly needs issue_context and contains expression-valued staged handler configuration
• generated activation installs .github/skills/issue-triage through compiler-managed skill installation
• Vally 0.13.0 strict lint, layout validation, and 6-plan experiment discovery passed again
• layer diff contains only .github/workflows/issue-triage-agent.md and .github/workflows/issue-triage-agent.lock.yml
• no model-backed Vally trial or live issue mutation was run