[MB-0601][P0] Create unified CI workflow for lint/test/coverage

[FarisZR]

Summary

Create a single deterministic CI workflow that runs lint, tests, coverage, and quality artifacts.

Scope

• Add .github/workflows/ci.yml as the primary quality gate.
• Consolidate duplicated checks where possible.
• Ensure workflow output is clear and actionable.

Acceptance Criteria

• PR checks include one unified quality workflow.
• Workflow is stable and reproducible across PRs.
• Branch protections can use this workflow as a required check.

Test Requirements (Mandatory)

• Validate pass/fail conditions with intentionally failing test and lint scenarios.

Dependencies

• MB-0501

Definition of Done

• CI workflow merged and required in branch protection.
• Existing workflows adjusted to avoid redundant quality checks.

References

• Plan: docs/plans/2026-02-21-refactor-long-term-modernization-backlog-plan.md
• Backlog ID: MB-0601