[P1] Add CI adapter maturity and contract notes for GitHub, GitLab, and Jenkins

[a-a-k]

Context

GitHub, GitLab, and Jenkins adapters wrap upstream Bering and Sheaft tools and exchange artifacts through v1alpha1 payload shapes. The adapter maturity story is not explicit enough for consumers.

Scope

Publish adapter maturity and compatibility notes per CI system.

Definition of Done

• Docs identify stable v1alpha1 fields and paths for each adapter.
• Report paths, artifact names, dotenv outputs, and gate decision fields are documented consistently.
• Docs state the migration policy for future adapter contract field changes.
• Adapter e2e validation is linked from the maturity notes.

[a-a-k]

Partial progress update (2026-05-01): 454971b fix: validate adapter gate decisions completed a technical prerequisite for this task.

What changed:

• GitHub Actions, GitLab component, Jenkins shared-library paths now validate gate decisions against the supported set: fail, pass, report, review, warn.
• Adapter e2e now includes a negative invalid-decision case.

Verification:

• python scripts/tasks.py e2e-adapters
• python scripts/tasks.py validate

Remaining scope for this issue: publish the operator-facing maturity/contract notes for each adapter, not just enforce the contract in code.

[a-a-k]

Release update (2026-05-01): mb3r-stack v0.3.1 shipped with adapter contract validation, but this issue remains open for the operator-facing maturity notes.

Relevant shipped commit:

• 454971b fix: validate adapter gate decisions

Release evidence:

• Release: https://github.com/MB3R-Lab/mb3r-stack/releases/tag/v0.3.1
• Workflow: https://github.com/MB3R-Lab/mb3r-stack/actions/runs/25205623771
• Local pre-release verification included python scripts/tasks.py e2e-adapters and python scripts/tasks.py release-dry-run.

Remaining scope: publish adapter maturity and contract notes for GitHub, GitLab, and Jenkins so operators know what is stable, preview, and intentionally out of scope.