Add documentation for running trunk branch bug test

Author: ammar-agent

tests/ipcMain/RUN_TRUNK_BRANCH_TEST.md

@@ -0,0 +1,79 @@
+# Running the Trunk Branch Bug Test
+
+This document describes how to run the test that demonstrates the trunk branch bug in `createWorkspace`.
+
+## Bug Description
+
+When creating a new workspace, the `trunkBranch` parameter should specify which branch to create the new branch from. However, in SSHRuntime, this parameter is being ignored and new branches are created from HEAD instead.
+
+**Location of bug:** `src/runtime/SSHRuntime.ts:592` and `618`
+- Line 592: `trunkBranch` is destructured as `_trunkBranch` (underscore indicates unused)
+- Line 618: Branch is created from `HEAD` instead of using `trunkBranch`
+
+## Test Location
+
+The test is located in `tests/ipcMain/createWorkspace.test.ts` in the "Branch handling" describe block within the runtime matrix.
+
+Test name: `"creates new branch from specified trunk branch, not from default branch"`
+
+## Running the Test
+
+### Prerequisites
+
+1. Set the `TEST_INTEGRATION` environment variable to enable integration tests:
+   ```bash
+   export TEST_INTEGRATION=1
+   ```
+
+2. Ensure Docker is installed and running (required for SSH runtime tests)
+
+### Run the specific test
+
+```bash
+# Run just this test for both runtimes
+./node_modules/.bin/jest tests/ipcMain/createWorkspace.test.ts -t "creates new branch from specified trunk branch"
+```
+
+Or using make:
+```bash
+TEST_INTEGRATION=1 make test
+```
+
+## Expected Results
+
+### LocalRuntime (PASS ✓)
+The test should **PASS** for LocalRuntime because it correctly uses the `trunkBranch` parameter when creating a new branch via `git worktree add -b`.
+
+### SSHRuntime (FAIL ✗)
+The test should **FAIL** for SSHRuntime because it ignores the `trunkBranch` parameter and creates branches from `HEAD` instead.
+
+The failure will manifest as:
+- `trunk-file.txt` will NOT exist (it should exist if branch was created from custom-trunk)
+- The test assertion `expect(checkOutput.trim()).toBe("exists")` will fail
+
+## Test Scenario
+
+The test creates the following git structure:
+
+```
+main (initial) ← custom-trunk (+ trunk-file.txt) 
+                           ↑
+                           Should branch from here
+                           
+main (initial) ← other-branch (+ other-file.txt)
+                           ↑
+                           HEAD might be here (bug)
+```
+
+When creating a workspace with `trunkBranch: "custom-trunk"`:
+- **Expected:** New branch contains `trunk-file.txt` (from custom-trunk)
+- **Actual (bug):** New branch might be from HEAD/default, missing `trunk-file.txt`
+
+## Test Coverage
+
+This test is part of the runtime matrix and runs for both:
+- `{ type: "local" }` - LocalRuntime
+- `{ type: "ssh" }` - SSHRuntime
+
+This ensures parity between runtime implementations.
+