feat: monorepo support — per-package .bec/rules.yaml (or document the pattern) #82

Context

Today there is exactly one .bec/rules.yaml, at the repo root. In a monorepo, teams own packages and will want to own their rules (packages/api/.bec/rules.yaml), with different severities per package.

The current workaround — scope rules with paths: ["packages/api/**"] globs — works and is now documented in usage.md's Performance and limits (#77), but it centralizes ownership at the root.

Options to evaluate

1. Discover nested .bec/rules.yaml files and merge, with the nearest file winning for its subtree (mirrors .gitignore layering). Portability question: what does export mean for a nested rule?
2. A root-level include: listing package rule files — explicit, no discovery cost.
3. Do nothing and bless the glob-scoping pattern as the monorepo answer in the docs.

Option 3 is free; 1 is the most ergonomic but touches the frozen file discovery contract, so it needs the deprecation lens even though it is additive.

Acceptance

• A decision recorded (even if it is option 3)
• If 1/2: tests for precedence, why, export/import and the hook path
• Docs updated either way