feat: add caching for generate command

[machado144]

Summary

• Cache GitHub-discovered repos and template variables in ~/.yankrun/cache.yaml
• Dry runs (--dryRun) use cached data to show placeholders without re-cloning
• Add --noCache flag to bypass cache and fetch fresh data
• Cache is automatically updated after each full generate run

What's cached

• GitHub repos: keyed by a SHA of your GitHub config (user, orgs, topic, prefix). Changes to config auto-invalidate.
• Template variables: keyed by repo URL + branch. Stores the git SHA and placeholder counts from the last clone.

New files

• domain/cache.go - cache data types
• services/cache.go - cache I/O, lookup, update, config hashing
• services/cache_test.go - 16 unit tests covering round-trip, lookup, update, SHA determinism

Test plan

• All 16 new cache unit tests pass
• All 24 existing unit tests pass (services package)
• All 10 integration tests pass
• go vet ./... clean
• go build ./... clean