fix: contain recipe template includes

[jbg]

Summary

• confine MiniJinja include and extends targets to the recipe directory
• reject absolute paths, parent traversal, and symlink escapes at the shared template loader
• preserve missing-template behavior and valid sibling or nested inheritance
• add focused absolute-path, traversal, symlink, and legitimate nested-template regressions

This restores the invariant that loading an untrusted recipe cannot make template inheritance read files outside that recipe's directory. It addresses the confirmed private audit finding project-loupe/audit-goose#91 without publishing exploit-specific content.

Validation

• Regression proof before the fix: 3 escape tests failed; the legitimate nested-template test passed
• cargo fmt --all
• cargo test -p goose recipe::template_recipe::tests::template_loader_tests — 4 passed
• cargo test -p goose recipe::build_recipe::tests::test_template_inheritance — 1 passed
• cargo build -p goose
• cargo clippy -p goose --all-targets -- -D warnings

Limitations

This change intentionally limits inheritance to existing UTF-8 template files beneath the canonical recipe directory. It does not change recipe discovery, YAML parsing, or template parameter behavior.

This finding was discovered by Project Loupe.