docs: Vite Task cache reuse and tool-reported caching

[wan9chi]

Summary

• Add an experimental guide for reusing Vite Task cache with GitHub Actions cache.
• Add a standalone Tool-Reported Caching guide under Execute and link it from task caching, CI caching, and run config docs.
• Clarify dependsOn, env, untrackedEnv, input, and output reference behavior needed by the cache examples.

Validation

• vp fmt docs/.vitepress/config.mts docs/config/run.md docs/guide/cache.md docs/guide/ci.md docs/guide/github-actions-cache.md docs/guide/run.md docs/guide/tool-reported-caching.md --check
• git diff --check
• pnpm -C docs build
• Rendered HTML anchor check for untrackedEnv
• Implementation checks against packages/cli/src/run-config.ts and packages/cli/binding/src/cli/resolver.rs
• GitHub cache behavior checked against actions/cache v6.1.0 docs

Notes

The GitHub Actions cache guide is marked experimental. I did not run a live GitHub Actions cache workflow for this branch.

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