feat: add pages_dir config and output_dir safety guard

[lucasnevespereira]

Summary

Two related changes around build paths:

1. Guard against unsafe output_dir. Previous behavior would os.RemoveAll whatever path you pointed output_dir at — setting it to . would delete the entire project on first build. New validateOutput() rejects:

   • Empty values
   • output_dir equal to the project root
   • output_dir outside the project tree (../escape)

2. Configurable pages_dir. Was hardcoded to pages/. Now overridable via ink.yaml:

pages_dir: src/pages

Defaults to pages/. Useful when migrating from layouts that already have a src/ convention.

Test plan

• go vet ./... clean
• go build ./... clean
• Existing examples still build (minimal, multi-page, library, bio)
• Guard rejects output_dir: "." with a clear error
• pages_dir: src resolves correctly when content lives at src/<page>/content.md