fix: use sh instead of /bin/sh as shell fallback on Windows (#891)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies
> - Agents run shell commands during workspace provisioning (git
worktree creation, runtime services)
> - When `process.env.SHELL` is unset, the code falls back to `/bin/sh`
> - But on Windows with Git Bash, `/bin/sh` doesn't exist as an absolute
path — Git Bash provides `sh` on PATH instead
> - This causes `child_process.spawn` to throw `ENOENT`, crashing
workspace provisioning on Windows
> - This PR extracts a `resolveShell()` helper that uses `$SHELL` when
set, falls back to `sh` (bare) on Windows or `/bin/sh` on Unix
> - The benefit is that agents running on Windows via Git Bash can
provision workspaces without shell resolution errors
## Summary
- `workspace-runtime.ts` falls back to `/bin/sh` when
`process.env.SHELL` is unset
- On Windows, `/bin/sh` doesn't exist → `spawn /bin/sh ENOENT`
- Fix: extract `resolveShell()` helper that uses `$SHELL` when set,
falls back to `sh` on Windows (Git Bash PATH lookup) or `/bin/sh` on
Unix

Three call sites updated to use the new helper.

Fixes #892

## Root cause

When Paperclip spawns shell commands in workspace operations (e.g., git
worktree creation), it uses `process.env.SHELL` if set, otherwise
defaults to `/bin/sh`. On Windows with Git Bash, `$SHELL` is typically
unset and `/bin/sh` is not a valid path — Git Bash provides `sh` on PATH
but not at the absolute `/bin/sh` location. This causes
`child_process.spawn` to throw `ENOENT`.

## Approach

Rather than hard-coding a Windows-specific absolute path (e.g.,
`C:\Program Files\Git\bin\sh.exe`), we use the bare `"sh"` command which
relies on PATH resolution. This works because:
1. Git Bash adds its `usr/bin` directory to PATH, making `sh` resolvable
2. On Unix/macOS, `/bin/sh` remains the correct default (it's the POSIX
standard location)
3. `process.env.SHELL` takes priority when set, so this only affects the
fallback

## Test plan

- [x] 7 unit tests for `resolveShell()`: SHELL set, trimmed, empty,
whitespace-only, linux/darwin/win32 fallbacks
- [x] Run a workspace provision command on Windows with `git_worktree`
strategy
- [x] Verify Unix/macOS is unaffected

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Paperclip <noreply@paperclip.ing>
Co-authored-by: Devin Foley <devin@devinfoley.com>

server/src/__tests__/workspace-runtime.test.ts

@@ -24,6 +24,7 @@ import {
   realizeExecutionWorkspace,
   releaseRuntimeServicesForRun,
   resetRuntimeServicesForTests,
+  resolveShell,
   sanitizeRuntimeServiceBaseEnv,
   stopRuntimeServicesForExecutionWorkspace,
   type RealizedExecutionWorkspace,
@@ -1345,6 +1346,60 @@ describe("ensureRuntimeServicesForRun", () => {
   });
 });
 
+describe("resolveShell (shell fallback)", () => {
+  const originalShell = process.env.SHELL;
+  const originalPlatform = process.platform;
+
+  afterEach(() => {
+    if (originalShell !== undefined) {
+      process.env.SHELL = originalShell;
+    } else {
+      delete process.env.SHELL;
+    }
+    Object.defineProperty(process, "platform", { value: originalPlatform });
+  });
+
+  it("returns process.env.SHELL when set", () => {
+    process.env.SHELL = "/usr/bin/zsh";
+    expect(resolveShell()).toBe("/usr/bin/zsh");
+  });
+
+  it("trims whitespace from SHELL env var", () => {
+    process.env.SHELL = "  /usr/bin/fish  ";
+    expect(resolveShell()).toBe("/usr/bin/fish");
+  });
+
+  it("falls back to /bin/sh on non-Windows when SHELL is unset", () => {
+    delete process.env.SHELL;
+    Object.defineProperty(process, "platform", { value: "linux" });
+    expect(resolveShell()).toBe("/bin/sh");
+  });
+
+  it("falls back to sh (bare) on Windows when SHELL is unset", () => {
+    delete process.env.SHELL;
+    Object.defineProperty(process, "platform", { value: "win32" });
+    expect(resolveShell()).toBe("sh");
+  });
+
+  it("falls back to /bin/sh on darwin when SHELL is unset", () => {
+    delete process.env.SHELL;
+    Object.defineProperty(process, "platform", { value: "darwin" });
+    expect(resolveShell()).toBe("/bin/sh");
+  });
+
+  it("treats empty SHELL as unset and uses platform fallback", () => {
+    process.env.SHELL = "";
+    Object.defineProperty(process, "platform", { value: "linux" });
+    expect(resolveShell()).toBe("/bin/sh");
+  });
+
+  it("treats whitespace-only SHELL as unset and uses platform fallback", () => {
+    process.env.SHELL = "   ";
+    Object.defineProperty(process, "platform", { value: "win32" });
+    expect(resolveShell()).toBe("sh");
+  });
+});
+
 describeEmbeddedPostgres("workspace runtime startup reconciliation", () => {
   let db!: ReturnType<typeof createDb>;
   let tempDb: Awaited<ReturnType<typeof startEmbeddedPostgresTestDatabase>> | null = null;