0010 - FactStore Languages

Context

The model.Rule struct has an AppliesTo.Languages field (declared in the rule YAML schema since day one), but the rule engine ignores it — every rule runs against every repo. This causes false signals: a pure Go repo gets penalized by P3.EXP.001 for not having .env.example, even though .env is not a Go convention.

The data needed to implement filtering already exists: RepoFacts.Languages is a map[string]int populated by the filesystem collector. It just isn't exposed on the FactStore interface.

Decision

1. Add Languages() map[string]int to model.FactStore. This is a public-interface change, requiring this ADR per CLAUDE.md §9.

2. The rule engine (rule.Engine.Evaluate) skips a rule when its AppliesTo.Languages is non-empty and none of the listed languages appear in facts.Languages(). Skipped rules are tracked in EvalResult.SkippedRuleIDs.

3. score.Compute accepts an optional skippedRuleIDs variadic. Skipped rules' weights are excluded from the denominator so they cannot penalize the score for ecosystems they don't target.

4. The code-generation tool (genrules) emits AppliesTo when the YAML declares applies_to.languages.

Consequences

• All FactStore implementations (real, test fakes) must add the Languages() method. Test fakes return nil.
• Rules without applies_to continue to run everywhere (backward compatible).
• Adding language tags to a rule is a YAML-only change + make generate.
• A single-language repo no longer gets penalized by rules targeting other ecosystems.