Expand tilde in LocalRuntime srcBaseDir at construction time

Previously, LocalRuntime would store srcBaseDir paths with tildes unexpanded,
leading to incorrect path construction when using path.join(). This simplifies
logic by ensuring all local paths are fully expanded at the IPC boundary.

Changes:
- Add expandTilde() utility for local file system path expansion
- Update LocalRuntime constructor to expand tilde in srcBaseDir
- Add comprehensive tests for both utilities

For SSH runtime, tilde paths are preserved and expanded using expandTildeForSSH()
when constructing remote commands, which is the correct behavior for remote paths.

Author: ammar-agent

src/runtime/LocalRuntime.test.ts

@@ -0,0 +1,31 @@
+import { describe, expect, it } from "bun:test";
+import * as os from "os";
+import * as path from "path";
+import { LocalRuntime } from "./LocalRuntime";
+
+describe("LocalRuntime constructor", () => {
+  it("should expand tilde in srcBaseDir", () => {
+    const runtime = new LocalRuntime("~/workspace");
+    const workspacePath = runtime.getWorkspacePath("/home/user/project", "branch");
+    
+    // The workspace path should use the expanded home directory
+    const expected = path.join(os.homedir(), "workspace", "project", "branch");
+    expect(workspacePath).toBe(expected);
+  });
+
+  it("should handle absolute paths without expansion", () => {
+    const runtime = new LocalRuntime("/absolute/path");
+    const workspacePath = runtime.getWorkspacePath("/home/user/project", "branch");
+    
+    const expected = path.join("/absolute/path", "project", "branch");
+    expect(workspacePath).toBe(expected);
+  });
+
+  it("should handle bare tilde", () => {
+    const runtime = new LocalRuntime("~");
+    const workspacePath = runtime.getWorkspacePath("/home/user/project", "branch");
+    
+    const expected = path.join(os.homedir(), "project", "branch");
+    expect(workspacePath).toBe(expected);
+  });
+});

src/runtime/LocalRuntime.ts

@@ -22,6 +22,7 @@ import { checkInitHookExists, getInitHookPath, createLineBufferedLoggers } from
 import { execAsync } from "../utils/disposableExec";
 import { getProjectName } from "../utils/runtime/helpers";
 import { getErrorMessage } from "../utils/errors";
+import { expandTilde } from "./tildeExpansion";
 
 /**
  * Local runtime implementation that executes commands and file operations
@@ -31,7 +32,8 @@ export class LocalRuntime implements Runtime {
   private readonly srcBaseDir: string;
 
   constructor(srcBaseDir: string) {
-    this.srcBaseDir = srcBaseDir;
+    // Expand tilde to actual home directory path for local file system operations
+    this.srcBaseDir = expandTilde(srcBaseDir);
   }
 
   async exec(command: string, options: ExecOptions): Promise<ExecStream> {

src/runtime/tildeExpansion.test.ts

@@ -0,0 +1,33 @@
+import { describe, expect, it } from "bun:test";
+import * as os from "os";
+import * as path from "path";
+import { expandTilde } from "./tildeExpansion";
+
+describe("expandTilde", () => {
+  it("should expand ~ to home directory", () => {
+    const result = expandTilde("~");
+    expect(result).toBe(os.homedir());
+  });
+
+  it("should expand ~/path to home directory + path", () => {
+    const result = expandTilde("~/workspace");
+    expect(result).toBe(path.join(os.homedir(), "workspace"));
+  });
+
+  it("should leave absolute paths unchanged", () => {
+    const absolutePath = "/abs/path/to/dir";
+    const result = expandTilde(absolutePath);
+    expect(result).toBe(absolutePath);
+  });
+
+  it("should leave relative paths unchanged", () => {
+    const relativePath = "relative/path";
+    const result = expandTilde(relativePath);
+    expect(result).toBe(relativePath);
+  });
+
+  it("should handle nested paths correctly", () => {
+    const result = expandTilde("~/workspace/project/subdir");
+    expect(result).toBe(path.join(os.homedir(), "workspace/project/subdir"));
+  });
+});

src/runtime/tildeExpansion.ts

@@ -1,11 +1,42 @@
 /**
- * Utilities for handling tilde path expansion in SSH commands
+ * Utilities for handling tilde path expansion
  *
- * When running commands over SSH, tilde paths need special handling:
+ * For SSH commands, tilde paths need special handling:
  * - Quoted tildes won't expand: `cd '~'` fails, but `cd "$HOME"` works
  * - Must escape special shell characters when using $HOME expansion
+ *
+ * For local paths, tildes should be expanded to actual file system paths.
  */
 
+import * as os from "os";
+import * as path from "path";
+
+/**
+ * Expand tilde to actual home directory path for local file system operations.
+ *
+ * Converts:
+ * - "~" → "/home/user" (actual home directory)
+ * - "~/path" → "/home/user/path"
+ * - "/abs/path" → "/abs/path" (unchanged)
+ *
+ * @param filePath - Path that may contain tilde prefix
+ * @returns Fully expanded absolute path
+ *
+ * @example
+ * expandTilde("~")           // => "/home/user"
+ * expandTilde("~/workspace") // => "/home/user/workspace"
+ * expandTilde("/abs/path")   // => "/abs/path"
+ */
+export function expandTilde(filePath: string): string {
+  if (filePath === "~") {
+    return os.homedir();
+  } else if (filePath.startsWith("~/")) {
+    return path.join(os.homedir(), filePath.slice(2));
+  } else {
+    return filePath;
+  }
+}
+
 /**
  * Expand tilde path to $HOME-based path for use in SSH commands.
  *