Add tests for the step that excludes .claude, .agents and AGENTS.md from git
in the new project: the three paths are ignored and absent from git status
after a run, while framework, .gitmodules and docs/artifact-registry.md stay
visible; no .gitignore is written and nothing is committed; a second run
writes each entry and the comment once and keeps the existing lines, also
when the file has no final newline; nothing is written for a path git
already ignores; a tracked path stays tracked and is named; the step is
skipped without the framework; the dry-run plan lists it.
The existing .env exclusion test also checks its comment line.
Refs #68
Task: MIL-009#4
Tirsvad