fix: remove unstaging logic from patch application

Fix critical bug in patcher.py where patches failed to apply to staged
files during pre-commit hooks.

**Root Cause:**
The apply_patch() function was unstaging files before applying patches:
1. File gets unstaged (git reset HEAD)
2. Patch tries to apply with --index flag
3. But patch was generated from STAGED content
4. Base state mismatch causes patch application to fail
5. Original changes get re-staged, AI changes are lost

**The Fix:**
Remove the unstaging logic entirely (lines 599-610, 639-641).
- Patches are generated from staged content (git diff --cached)
- The --index flag correctly applies to both working tree and index
- No need to unstage first - that changes the base state

**Changes:**
- Deleted 19 lines of problematic unstaging code
- Added clear comment explaining why unstaging is harmful
- Simplified apply_patch() function

**Impact:**
- Patches now apply correctly during pre-commit hooks
- Status changes (OPEN → READY_FOR_DESIGN) work properly
- Gate creation (design_gate_writer) will trigger correctly
- No behavior change for non-staged files

**Testing:**
- All 18 existing tests still pass
- Bundle rebuilt and verified

Discovered during end-to-end testing when AI-generated status promotion
patches failed with "Failed to apply patch (strict and 3-way both failed)".

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

Co-Authored-By: Claude <noreply@anthropic.com>

automation/patcher.py

@@ -596,20 +596,11 @@ def apply_patch(repo_root: Path, patch_file: Path, patch_level: str, output_rel:
     """
     absolute_patch = patch_file.resolve()
 
-    # Check if the output file is currently staged
+    # Note: Patches are generated from staged content (git diff --cached).
-    # If it is, unstage it so the patch applies to the working directory version
+    # The --index flag applies patches to both working tree and index, which is
-    was_staged = False
+    # what we want. Do NOT unstage files before applying - that changes the base
-    check_staged = run(
+    # state and causes patch application to fail.
-        ["git", "diff", "--cached", "--name-only", "--", output_rel.as_posix()],
-        cwd=repo_root,
-        check=False
-    )
-    if check_staged.returncode == 0 and check_staged.stdout.strip():
-        # File is staged, unstage it temporarily
-        was_staged = True
-        run(["git", "reset", "HEAD", "--", output_rel.as_posix()], cwd=repo_root, check=False)
 
-    try:
     # First, try a dry-run check with --check to see if the patch applies cleanly.
     check_args = ["git", "apply", patch_level, "--index", "--check", absolute_patch.as_posix()]
     if run(check_args, cwd=repo_root, check=False).returncode == 0:
@@ -635,10 +626,6 @@ def apply_patch(repo_root: Path, patch_file: Path, patch_level: str, output_rel:
 
     # If all attempts fail, raise an error.
     raise PatchGenerationError("Failed to apply patch (strict and 3-way both failed)")
-    finally:
-        # If the file was staged before, re-stage it now (whether patch succeeded or failed)
-        if was_staged:
-            run(["git", "add", "--", output_rel.as_posix()], cwd=repo_root, check=False)
 
 
 def run(args: list[str], cwd: Path, check: bool = True) -> subprocess.CompletedProcess[str]: