chore(skill): prune dead local-filesystem helpers left over from Box migration

Follow-up to the Box-only refactor. The previous commit removed the
local-fallback BRANCHES from every public method; this one removes the
HELPERS those branches called, which are now unreachable.

SkillService (service/skill.py): 787 → 449 lines
  Removed: scan_directory (sync), _read_skill_package, _write_skill_md,
  _resolve_create_field, _managed_skill_path,
  _managed_install_root_for_package, _normalize_package_root,
  _resolve_skill_path, _find_skill_entry, _discover_skill_directories,
  _safe_extract_zip, _extract_uploaded_skill_to_temp,
  _download_github_skill_to_temp, _resolve_github_source_root,
  _build_preview_target_dir, _preview_skill_candidates,
  _select_preview_candidates, _install_preview_candidates,
  _preview_source_root, _resolve_installed_skills, plus the
  module-level _FRONTMATTER_FIELDS and _build_skill_md.
  Kept (still needed by the surviving GitHub-import path):
  _download_github_asset, _download_github_skill_directory_as_zip,
  _find_github_skill_archive_entry, _copy_github_skill_directory_to_zip,
  _is_github_skill_md_url, _parse_github_skill_md_url,
  _resolve_github_skill_md_package_name, _validate_github_asset_url,
  _uploaded_skill_target_stem, _validate_skill_name.
  Imports dropped: shutil, tempfile, yaml, ....utils.paths.

SkillManager (skill/manager.py): 187 → 88 lines
  Removed: get_managed_skills_root, _discover_skill_directories,
  _find_skill_entry, _load_skill_file, _normalize_package_root.
  Imports dropped: datetime, parse_frontmatter, paths.

Tests:
  - test_skill_service.py: drop the 3 sync scan_directory tests +
    skill_service fixture + _create_skill_file helper
  - test_skill_tools.py: drop test_load_skill_file_success; rename
    TestSkillManagerPackageLoading → TestSkillManagerCache

Full unit suite: 277 passed, 1 skipped. ``ruff check`` clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit_tests/provider/test_skill_tools.py

@@ -35,24 +35,10 @@ def _make_skill_data(
     }
 
 
-class TestSkillManagerPackageLoading:
-    def test_load_skill_file_success(self):
-        from langbot.pkg.skill.manager import SkillManager
-
-        ap = _make_ap()
-        mgr = SkillManager(ap)
-
-        with tempfile.TemporaryDirectory() as tmpdir:
-            skill_md = os.path.join(tmpdir, 'SKILL.md')
-            with open(skill_md, 'w', encoding='utf-8') as f:
-                f.write('---\ndescription: Test skill\n---\n\n# Test Skill\nDo things.')
-
-            skill_data = _make_skill_data(package_root=tmpdir)
-            result = mgr._load_skill_file(skill_data)
-
-            assert result is True
-            assert skill_data['instructions'] == '# Test Skill\nDo things.'
-            assert skill_data['description'] == 'Test skill'
+class TestSkillManagerCache:
+    """The Box runtime is the only source of truth — SkillManager just holds
+    an in-memory cache populated by ``reload_skills``. There is no local
+    filesystem reader anymore."""
 
     def test_refresh_skill_from_disk_reports_cache_presence(self):
         """Box is the only source of truth for skill content. refresh_skill_from_disk

tests/unit_tests/test_skill_service.py

@@ -6,69 +6,6 @@ import pytest
 from langbot.pkg.api.http.service.skill import SkillService
 
 
-def _create_skill_file(
-    path,
-    *,
-    name: str = 'imported-skill',
-    display_name: str = '',
-    description: str = 'Imported from local directory',
-    body: str = 'Skill instructions',
-) -> None:
-    frontmatter = ['name: ' + name, 'description: ' + description]
-    if display_name:
-        frontmatter.insert(1, 'display_name: ' + display_name)
-
-    path.write_text(
-        '---\n' + '\n'.join(frontmatter) + f'\n---\n\n{body}\n',
-        encoding='utf-8',
-    )
-
-
-@pytest.fixture
-def skill_service():
-    app = SimpleNamespace(
-        skill_mgr=SimpleNamespace(
-            refresh_skill_from_disk=lambda *_args, **_kwargs: True,
-            reload_skills=AsyncMock(),
-        )
-    )
-    return SkillService(app)
-
-
-def test_scan_directory_supports_nested_skill_within_two_levels(skill_service, tmp_path):
-    nested_dir = tmp_path / 'downloaded' / 'self-improving-agent'
-    nested_dir.mkdir(parents=True)
-    _create_skill_file(nested_dir / 'SKILL.md')
-
-    result = skill_service.scan_directory(str(tmp_path))
-
-    assert result['package_root'] == str(nested_dir.resolve())
-    assert result['entry_file'] == 'SKILL.md'
-    assert result['name'] == 'imported-skill'
-    assert result['instructions'] == 'Skill instructions'
-
-
-def test_scan_directory_rejects_ambiguous_nested_skill_directories(skill_service, tmp_path):
-    first_dir = tmp_path / 'skills' / 'alpha'
-    second_dir = tmp_path / 'skills' / 'beta'
-    first_dir.mkdir(parents=True)
-    second_dir.mkdir(parents=True)
-    _create_skill_file(first_dir / 'SKILL.md', body='alpha instructions')
-    _create_skill_file(second_dir / 'SKILL.md', body='beta instructions')
-
-    with pytest.raises(ValueError, match='Multiple skill directories found'):
-        skill_service.scan_directory(str(tmp_path))
-
-
-def test_scan_directory_errors_when_skill_is_deeper_than_two_levels(skill_service, tmp_path):
-    deep_dir = tmp_path / 'a' / 'b' / 'c'
-    deep_dir.mkdir(parents=True)
-    _create_skill_file(deep_dir / 'SKILL.md')
-
-    with pytest.raises(ValueError, match='max depth: 2'):
-        skill_service.scan_directory(str(tmp_path))
-
-
 class TestRequireBoxForWrite:
     """Box is the only source of truth for skills — there is no local
     filesystem fallback. Every write and (most) read methods refuse cleanly