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).

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

strix/llm/multi_provider_setup.py

@@ -6,10 +6,6 @@ route so models named ``anthropic/<model>`` go through
 on the system message). Every other prefix
 (``openai/`` / ``gemini/`` / ``openrouter/`` / ``litellm/...``) falls
 through to the SDK's built-in litellm routing.
-
-References:
-    - PLAYBOOK.md §2.7
-    - AUDIT_R3.md C17 (model alias validation; raise UserError on bad alias)
 """
 
 from __future__ import annotations