Problem: Typos in forge refs like `gh:user/repo#42` silently persist —
there's no feedback when a ref points to a nonexistent issue.
Solution: Add `forge.validate` config option. When enabled, `diff.apply()`
returns new/changed `ForgeRef[]` and `forge.validate_refs()` fetches
metadata for each, logging specific warnings for not-found, auth, or
CLI-missing errors.