refactor: scrub migration scars, dead code, and unused helpers

- Strip PLAYBOOK / AUDIT / Phase-N / C-numbered references from
  module docstrings across 16 files; rename
  ``_PHASE1_PARALLEL_DEFAULT`` → ``_PARALLEL_TOOL_CALLS_DEFAULT``.
- Delete unused exception classes: ``SandboxInitializationError``,
  ``ImplementedInClientSideOnlyError``.
- Delete the no-op ``on_handoff`` hook (we don't use SDK handoffs).
- Delete the unreachable backward-compat tab-delimited fallback in
  ``_parse_git_diff_output``.
- Delete orphaned ``strix/tools/load_skill/`` (dir contained only a
  pycache) and stale pycache files.
- Rewrite ``strix/skills/__init__.py``: 168 → 56 LoC. Drop seven
  helper functions (``get_available_skills``, ``get_all_skill_names``,
  ``validate_skill_names``, ``parse_skill_list``,
  ``validate_requested_skills``, ``generate_skills_description``,
  ``_get_all_categories``) — none had external callers; only
  ``load_skills`` is used.
- Drop the stale ``strix/agents/sdk_factory.py`` per-file ruff ignore
  (file no longer exists).

strix/interface/utils.py

@@ -733,18 +733,7 @@ def _parse_name_status_z(raw_output: bytes) -> list[DiffEntry]:
             index += 2
             continue
 
-        # Backward-compat fallback if output is tab-delimited unexpectedly.
-        status_fallback, has_tab, first_path = token.partition("\t")
-        if not has_tab:
-            break
-        fallback_code = status_fallback[:1]
-        fallback_similarity: int | None = None
-        if len(status_fallback) > 1 and status_fallback[1:].isdigit():
-            fallback_similarity = int(status_fallback[1:])
-        entries.append(
-            DiffEntry(status=fallback_code, path=first_path, similarity=fallback_similarity)
-        )
-        index += 1
+        break
 
     return entries