fix: prevent learning mode from collapsing read paths to $HOME

Files directly under ~ (e.g., ~/.gitignore, ~/.npmrc) were collapsed
to the home directory, defeating sandboxing. Now keeps exact file paths
when the parent directory would be $HOME.

internal/sandbox/learning.go

@@ -220,9 +220,15 @@ func CollapsePaths(paths []string) []string {
 		}
 	}
 
-	// For standalone paths, use their parent directory
+	// For standalone paths, use their parent directory — but never collapse to $HOME
 	for _, p := range standalone {
-		result = append(result, filepath.Dir(p))
+		parent := filepath.Dir(p)
+		if parent == home {
+			// Keep exact file path to avoid opening entire home directory
+			result = append(result, p)
+		} else {
+			result = append(result, parent)
+		}
 	}
 
 	// Sort and deduplicate (remove sub-paths of other paths)

internal/sandbox/learning_test.go

@@ -78,6 +78,7 @@ func TestCollapsePaths(t *testing.T) {
 		name        string
 		paths       []string
 		contains    []string // paths that should be in the result
+		notContains []string // paths that must NOT be in the result
 	}{
 		{
 			name: "multiple paths under same app dir",
@@ -111,6 +112,33 @@ func TestCollapsePaths(t *testing.T) {
 				"/home/testuser/.config/opencode",
 			},
 		},
+		{
+			name: "files directly under home stay as exact paths",
+			paths: []string{
+				"/home/testuser/.gitignore",
+				"/home/testuser/.npmrc",
+			},
+			contains: []string{
+				"/home/testuser/.gitignore",
+				"/home/testuser/.npmrc",
+			},
+			notContains: []string{"/home/testuser"},
+		},
+		{
+			name: "mix of home files and app dir paths",
+			paths: []string{
+				"/home/testuser/.gitignore",
+				"/home/testuser/.cache/opencode/db/main.sqlite",
+				"/home/testuser/.cache/opencode/version",
+				"/home/testuser/.npmrc",
+			},
+			contains: []string{
+				"/home/testuser/.gitignore",
+				"/home/testuser/.npmrc",
+				"/home/testuser/.cache/opencode",
+			},
+			notContains: []string{"/home/testuser"},
+		},
 	}
 
 	for _, tt := range tests {
@@ -134,6 +162,13 @@ func TestCollapsePaths(t *testing.T) {
 					t.Errorf("CollapsePaths() = %v, missing expected path %q", got, want)
 				}
 			}
+			for _, bad := range tt.notContains {
+				for _, g := range got {
+					if g == bad {
+						t.Errorf("CollapsePaths() = %v, should NOT contain %q", got, bad)
+					}
+				}
+			}
 		})
 	}
 }