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>
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