feat: helper to reduce GithubCDKPipeline boilerplate

[hoegertn]

What

Add a helper (e.g. taimosCdkPipeline(app, options)) that reduces the repeated GithubCDKPipeline boilerplate in every backend/infra sub-project.

Why

Every CDK sub-project across the fleet hand-writes the same GithubCDKPipeline shape:

• the iamRoleArns triple (default / assetPublishing / synth) all set to the same deploy-role ARN,
• the same versioning block (VersioningStrategy.commitCount() + VersioningOutputs.cloudFormationOnly()),
• useGithubEnvironments: true,
• a personalStage sandbox,
• paths scoped to the package.

See nornkeep/.projen/infra.ts, wir-platform/.projen/backend.ts, radball-digital/.projen/infra.ts — near-identical modulo the role ARN and stage list.

Scope / approach

• A thin helper taking { pipelineName, roleArn, paths, stages, personalStage? } and applying the standard versioning + environments + iamRoleArns fan-out.
• Keep it composable — it should return the GithubCDKPipeline so a consumer can still tweak it.
• Optional; lower priority than T1/T3/T5. Consider whether this belongs in @taimos/projen or as guidance in the scaffolding playbook, since it wraps a projen-pipelines type.

Acceptance criteria

• A backend pipeline can be declared with role ARN + stages and nothing else.
• Escape hatch to customize the returned pipeline.
• Test asserts the generated workflow.
• API.md regenerated.

Part of the taimos-projen review. Lower priority (optional polish).