test: add path boundary coverage

[CoderDeltaLAN]

Adds path boundary coverage for repository-relative discovery and init write behavior.

Scope:

• add tests/test_path_boundaries.py only;
• verify discovery returns repository-relative supported paths;
• verify discovery ignores backup and atomic temporary files;
• verify init planning targets root AGENTS.md only;
• verify init --write creates only root AGENTS.md;
• verify init --write backs up only root AGENTS.md;
• verify init --write leaves no atomic temporary file.

Validation:

• ./scripts/check.sh passed with 56 tests;
• direct boundary test module passed;
• check console output verified;
• check JSON output verified;
• check Markdown output verified;
• init --dry-run verified as read-only;
• init --write verified for create and backup behavior;
• no raw secret-like literals in changed files.