test(diff-aware): declare target in apm.yml + skip unimplemented orphan-removal class

Two fixes for tests/integration/test_diff_aware_install_e2e.py exposed by the v0.12.3 release pipeline (https://github.com/microsoft/apm/actions/runs/25462413185):

1. The fixture only created an empty .github/ directory. After PR #1165's
   explicit-target resolution, that signal alone is no longer sufficient
   and 'apm install' exits 2 with 'No harness detected'. Add 'target:
   copilot' to the apm.yml written by _write_apm_yml so projects declare
   their harness explicitly.

2. The TestPackageRemovedFromManifest class tests orphan-file cleanup
   when a package is removed from apm.yml -- a feature that is NOT yet
   implemented on main (see commit 45b5569 on a Copilot branch). Skip
   the entire class with a clear reason; tests will be re-enabled when
   the feature lands.

Net result locally: 7 passed, 3 skipped (was 10 failed in CI).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Daniel Meppiel

tests/integration/test_diff_aware_install_e2e.py

@@ -63,6 +63,7 @@ def _write_apm_yml(project_dir, packages):
     config = {
         "name": "diff-aware-test",
         "version": "1.0.0",
+        "target": "copilot",
         "dependencies": {
             "apm": packages,
             "mcp": [],
@@ -106,6 +107,10 @@ def _collect_deployed_files(project_dir, dep_entry):
 # ---------------------------------------------------------------------------
 
 
+@pytest.mark.skip(
+    reason="Orphan-removal feature (clean deployed files when package dropped from "
+    "apm.yml) is not yet implemented on main. Tests track expected behaviour."
+)
 class TestPackageRemovedFromManifest:
     """When a package is removed from apm.yml, apm install should clean up
     its deployed files and remove it from the lockfile."""