Add gitignore-claude template and wire into registration flows

Introduce scripts/project_rules/gitignore-claude.template with the fleet
pattern that tracks .claude/rules/ while ignoring machine-local Claude state.
ensure_gitignore_claude_rules.py applies it during statehub register,
register_project.sh, and update_agent_instruction_files fleet regen.

scripts/ensure_gitignore_claude_rules.py

@@ -0,0 +1,63 @@
+"""Ensure repos track .claude/rules/ while ignoring other Claude Code local state."""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parent.parent
+SNIPPET_PATH = ROOT / "scripts" / "project_rules" / "gitignore-claude.template"
+MARKER = "# state-hub: track .claude/rules"
+TRACK_RULE = "!.claude/rules/"
+BLANKET_IGNORE_RE = re.compile(r"^\.claude/?\s*$", re.MULTILINE)
+
+
+def load_snippet() -> str:
+    text = SNIPPET_PATH.read_text(encoding="utf-8").strip()
+    return f"{MARKER}\n{text}\n"
+
+
+def snippet_present(text: str) -> bool:
+    return TRACK_RULE in text
+
+
+def ensure_claude_gitignore(gitignore_path: Path) -> bool:
+    """Return True when the file was created or modified."""
+    snippet = load_snippet()
+    if gitignore_path.exists():
+        text = gitignore_path.read_text(encoding="utf-8")
+        if snippet_present(text):
+            return False
+        if BLANKET_IGNORE_RE.search(text):
+            updated = BLANKET_IGNORE_RE.sub(snippet.rstrip(), text, count=1)
+            if updated != text:
+                gitignore_path.write_text(_ensure_trailing_newline(updated), encoding="utf-8")
+                return True
+        separator = "" if text.endswith("\n") or not text else "\n"
+        gitignore_path.write_text(f"{text}{separator}\n{snippet}", encoding="utf-8")
+        return True
+
+    gitignore_path.write_text(snippet, encoding="utf-8")
+    return True
+
+
+def _ensure_trailing_newline(text: str) -> str:
+    return text if text.endswith("\n") else f"{text}\n"
+
+
+def main() -> int:
+    import argparse
+
+    parser = argparse.ArgumentParser(description="Ensure .gitignore tracks .claude/rules/.")
+    parser.add_argument("path", type=Path, help="Repo root or .gitignore file path")
+    args = parser.parse_args()
+
+    target = args.path
+    gitignore = target if target.name == ".gitignore" else target / ".gitignore"
+    changed = ensure_claude_gitignore(gitignore)
+    print(f"{'updated' if changed else 'ok'}: {gitignore}")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())