Add workflow integrations for CI, static site generators, and stale-output regeneration

[mintyPT]

Problem

Users who generate images at scale need ClickClick to fit cleanly into existing build systems. Today the project has examples and scripts, but not a polished integration story.

Proposal

Add official workflows for CI and common static-site or app pipelines.

Acceptance criteria

• Add GitHub Actions examples for rendering and validating images.
• Add docs for Next.js, Astro, and generic static-site workflows.
• Add a stale-output mode that regenerates only changed inputs where possible.
• Add watch support for config-driven sets, not only single templates.
• Ensure cache behavior is documented for CI and local development.
• Add examples for package scripts and library usage.
• Add tests for stale detection and watch/config behavior where feasible.