feat(static): implement static file copy and passthrough

[zeroedin]

Summary

• Implement CopyStatic to walk and copy all files from static/ to output root, gracefully handling missing or empty directories
• Implement CopyPassthrough to copy files from external directories according to passthrough mappings, erroring on missing source paths
• Implement CopyPassthroughWithValidation to add overlap protection by silently skipping managed directories (content, layouts, assets, static, data)
• Add testdata fixtures for static, external, and empty directories

Test plan

• All 8 static package tests pass (go test ./internal/static/ -v)
• Review that CopyStatic handles empty/missing dirs without error
• Review that CopyPassthrough errors on nonexistent From paths
• Review that CopyPassthroughWithValidation silently ignores managed directories

🤖 Generated with Claude Code

[zeroedin]

Code Review: feat(static): implement static file copy and passthrough

Clean implementation of a leaf package. 8/8 tests pass, go vet clean. The code is straightforward and correct for the test suite's expectations. A few spec compliance gaps to address.

Summary

Area	Verdict
CopyStatic	✅ Correct — walks, copies, handles missing/empty dirs gracefully
CopyPassthrough	⚠️ Ignores projectRoot param — paths not resolved per spec
CopyPassthroughWithValidation	⚠️ Managed dir matching is too shallow — misses relative/absolute variants
copyFile	⚠️ Doesn't preserve file permissions
Test fixtures	✅ Good — .gitignore in output dir, .gitkeep for empty dirs
Test output cleanup	⚠️ Tests leave artifacts in the worktree

See inline comments for details.