Support recursive directory scanning

[hermanngeorge15]

Problem

spec-linter check . only scans top-level files in the given directory. In a monorepo, CLAUDE.md files may be nested:

my-monorepo/
├── CLAUDE.md
├── packages/
│   ├── api/CLAUDE.md
│   └── frontend/CLAUDE.md

Running spec-linter check . misses the nested files.

Proposal

Add a --recursive flag (or make it the default) that walks subdirectories looking for spec files matching the known patterns (CLAUDE.md, GEMINI.md, AGENTS.md, etc.).

Acceptance Criteria

• spec-linter check . --recursive finds nested CLAUDE.md files
• Default behavior (non-recursive) is unchanged
• Respects .gitignore or node_modules exclusion

Labels

enhancement, cli