fix: prevent GitStore from creating nested repos and overwriting .gitignore (#2980)

GitStore.init() now checks if the workspace is already inside a git
repository before calling porcelain.init(). If so, it refuses to create
a nested repo. Additionally, existing .gitignore files are preserved
by appending only missing Dream-specific entries rather than overwriting.

Closes #2980

tests/utils/test_gitstore.py

@@ -1,7 +1,7 @@
 """Tests for GitStore — line_ages() and core git operations."""
 
 import time
-from datetime import datetime, timezone, timedelta
+from datetime import datetime, timedelta, timezone
 from unittest.mock import patch
 
 import pytest
@@ -89,3 +89,86 @@ class TestLineAges:
         # "- new" line and "- keep" line both age=0 (same day), but
         # the key point is we get per-line results
         assert len(ages) == 7
+
+
+class TestNestedRepoProtection:
+    """Regression tests for GitHub issue #2980: nested repo protection."""
+
+    def test_init_refuses_inside_git_repo(self, tmp_path):
+        """init() should detect it's inside an existing git repo and refuse."""
+        project = tmp_path / "project"
+        project.mkdir()
+        (project / ".git").mkdir()
+
+        workspace = project / "workspace"
+        workspace.mkdir()
+
+        g = GitStore(workspace, tracked_files=["MEMORY.md"])
+        result = g.init()
+
+        assert result is False
+        assert not (workspace / ".git").is_dir()
+
+    def test_init_preserves_existing_gitignore(self, tmp_path):
+        """init() should preserve existing .gitignore entries and append new ones."""
+        workspace = tmp_path / "workspace"
+        workspace.mkdir()
+
+        existing = "*.pyc\n__pycache__/\n"
+        (workspace / ".gitignore").write_text(existing, encoding="utf-8")
+
+        g = GitStore(workspace, tracked_files=["MEMORY.md"])
+        result = g.init()
+
+        assert result is True
+        gitignore = (workspace / ".gitignore").read_text(encoding="utf-8")
+        assert "*.pyc" in gitignore
+        assert "__pycache__/" in gitignore
+        assert "!MEMORY.md" in gitignore
+        assert "!.gitignore" in gitignore
+
+    def test_init_no_gitignore_creates_new(self, tmp_path):
+        """init() should create .gitignore with Dream content when none exists."""
+        workspace = tmp_path / "workspace"
+        workspace.mkdir()
+
+        g = GitStore(workspace, tracked_files=["MEMORY.md"])
+        result = g.init()
+
+        assert result is True
+        gitignore = (workspace / ".gitignore").read_text(encoding="utf-8")
+        expected = g._build_gitignore()
+        assert gitignore == expected
+
+    def test_init_gitignore_merge_idempotent(self, tmp_path):
+        """init() should not duplicate Dream entries already in .gitignore."""
+        workspace = tmp_path / "workspace"
+        workspace.mkdir()
+
+        # Pre-existing .gitignore that already has some Dream entries
+        existing = "*.pyc\n/*\n!MEMORY.md\n"
+        (workspace / ".gitignore").write_text(existing, encoding="utf-8")
+
+        g = GitStore(workspace, tracked_files=["MEMORY.md"])
+        result = g.init()
+
+        assert result is True
+        gitignore = (workspace / ".gitignore").read_text(encoding="utf-8")
+        # No duplicate lines
+        lines = gitignore.splitlines()
+        assert lines.count("/*") == 1
+        assert lines.count("!MEMORY.md") == 1
+        # Existing entry preserved, new Dream entries appended
+        assert "*.pyc" in gitignore
+        assert "!.gitignore" in gitignore
+
+    def test_init_outside_git_repo_works_normally(self, tmp_path):
+        """init() should succeed and create .git when not inside a git repo."""
+        workspace = tmp_path / "workspace"
+        workspace.mkdir()
+
+        g = GitStore(workspace, tracked_files=["MEMORY.md"])
+        result = g.init()
+
+        assert result is True
+        assert (workspace / ".git").is_dir()