docs: add configuration.md (skill test output), remove thin from-source.md
10 patterns, 989 lines. Full skill spec compliance:
- Source hyperlinks (commit SHA permalinks)
- Before/after code examples for every pattern
- Over-application warnings with code
- Anti-patterns with DON'T/DO blocks
- Decision tree at end
- Cross-references to related topic files
Patterns: zero-value config, options struct, functional options,
default instances, init-time registration, context values,
builder (anti-pattern), function fields, immutable-after-use, Clone.
Rodin
99c0865e93