Build Shared CI Workflow for Agent Runs

## Goal
Provide reusable workflow/action(s) for agent execution and validation.

## Why
Developers need one standard pipeline shape across repos with clear required checks and artifact outputs.

## Tasks
- Add reusable workflow for the mandatory Lane A path.
- Integrate stages: build -> test -> policy -> artifact publish.
- Normalize failure reasons and status names across repos.
- Document adoption steps for maintainers.
- Add optional Lane B integration hooks that never affect merge eligibility.

## Required Pipeline Outputs
- Policy decision artifact
- Test summary artifact
- Traceability/quality metrics artifact
- Run metadata (commit, lane, tool versions)

## Docs-as-code pilot tasks
- Implement workflow job sequence using existing traceability scripts.
- Upload metrics and gate outputs as artifacts.
- Expose check names intended for branch protection.

## Done When
- At least 2 pilot repos run the shared workflow end-to-end.
- Required check names are stable and documented.
- Lane B can be disabled without breaking merge-critical checks.

Parent: https://github.com/eclipse-score/score/issues/2827