feat(examples): add marketing-site, mobile-app, infra/devops, data-pipeline examples

[vivekchand]

Why

`examples/` currently has 5 starter files (cli-tool, library, web-service, frontend-app, ml-project). Three more stack archetypes deserve a tailored loop:

Examples to add

`examples/marketing-site.md`

• Inspiration: the FLYWHEEL.md just written for clawmetry-landing (clawmetry-landing/FLYWHEEL.md).
• Distinguishing rules: copy quality (plain over metaphor, no em-dashes), i18n source-of-truth + autotranslate, SEO as a first-class surface (sitemap + JSON-LD discipline), Cloud Run / Vercel / Netlify auto-deploy + smoke gate, analytics-driven Learn (A/B + funnel).
• Done-when each stage: "the change is live on the production domain in the affected locale, the analytics event fires, and a screenshot exists."

`examples/mobile-app.md`

• Distinguishing rules: store review as an explicit gate (Apple/Google review), staged rollout (1% → 10% → 100%) as a phased Ship, crash-free rate + ANR as evidence, force-update mechanism for hot bugs, App Store listing/screenshots as part of the loop.

`examples/infra-devops.md`

• Distinguishing rules: Terraform/IaC plan as a gate (apply needs sign-off for risky changes), blast radius classification (region/account/cell), runbook + on-call rotation as part of Learn, post-incident review feeds Improve, error-budget/SLO as the evidence bar.

`examples/data-pipeline.md`

• Distinguishing rules: data quality checks (Great Expectations / dbt tests) as the evidence bar, backfill strategy as Ship, schema migration gates, lineage tracking, cost per run + freshness SLO as Learn signals.

Acceptance

• Each file follows the existing example template (intro, the loop with adapted stages + Done-when + Gates, the bar adapted, license note).
• All four added in a single PR for symmetry.
• Linked from `examples/README.md`.