docs: add validated examples library demonstrating prompt schema and features

[PredictabilityAtScale]

Motivation

• Provide a small, teachable examples library that demonstrates each major PromptOpsKit feature and good prompt-authoring practices.
• Give users validation-safe, copy-pasteable prompt assets (including composition, context hardening, overrides, provider controls, and response schemas) so they can learn by example.

Description

• Add an examples/ folder with examples/README.md explaining layout, validation, and inspect/render usage.
• Add examples/prompts/defaults.md demonstrating folder-level defaults (provider, model, metadata, and cache hints) and two reusable include fragments in examples/prompts/shared/ for composition examples.
• Add eight vetted prompt examples (01-basic.md through 08-openai-responses.md) that cover interpolation, context validation, includes/composition, env/tier overrides, structured JSON responses, tools/MCP, provider cache/options/raw passthrough, and the openai-responses provider; each example includes a # Notes section describing the main points.
• Make schema- and validation-safe edits (per-prompt input validators and cache hints) so all examples validate without warnings or errors.

Testing

• Built the package artifacts with npm run build and the build completed successfully.
• Validated all example prompts with node dist/cli/index.js validate examples/prompts and the run reported Validated 10 file(s): 0 error(s), 0 warning(s).

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Codex Task