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/runtime/__init__.py

@@ -1,31 +1,13 @@
 """Strix runtime package.
 
-What lives here:
+- :class:`strix.runtime.strix_docker_client.StrixDockerSandboxClient` —
+  host-side ``DockerSandboxClient`` subclass that injects
+  ``NET_ADMIN`` / ``NET_RAW`` capabilities and ``host.docker.internal``
+  extra-hosts, used by the per-scan session manager
+  (:mod:`strix.sandbox.session_manager`).
 
-- :class:`StrixDockerSandboxClient` — host-side ``DockerSandboxClient``
-  subclass that injects ``NET_ADMIN`` / ``NET_RAW`` capabilities and
-  ``host.docker.internal`` extra-hosts, used by the per-scan session
-  manager (:mod:`strix.sandbox.session_manager`).
-
-- ``tool_server.py`` — the FastAPI server that runs *inside* the
-  sandbox container; sandbox-bound tools (browser, terminal, python,
-  file_edit, proxy) POST here from the host via
+- ``tool_server.py`` — FastAPI server that runs inside the sandbox
+  container. Sandbox-bound tools (browser, terminal, python, file_edit,
+  proxy) POST here from the host via
   :func:`strix.tools._sandbox_dispatch.post_to_sandbox`.
-
-The legacy DockerRuntime / AbstractRuntime + ``get_runtime`` /
-``cleanup_runtime`` globals were removed when the SDK harness took
-over scan lifecycle; sandbox sessions are now per-scan and managed by
-:func:`strix.sandbox.session_manager.create_or_reuse`.
 """
-
-
-class SandboxInitializationError(Exception):
-    """Raised when sandbox initialization fails (e.g., Docker issues)."""
-
-    def __init__(self, message: str, details: str | None = None):
-        super().__init__(message)
-        self.message = message
-        self.details = details
-
-
-__all__ = ["SandboxInitializationError"]

strix/runtime/strix_docker_client.py

@@ -11,13 +11,8 @@ additions before the final create call:
 These are required for raw-socket pentest tools (nmap -sS) and for letting
 the agent reach host-served apps via ``host.docker.internal``.
 
-Pinned to ``openai-agents==0.14.6``. Bumping the SDK version requires
-re-merging the parent body. Track upstream PR for an injection hook.
-
-References:
-    - PLAYBOOK.md §2.2
-    - AUDIT.md §2.3 (C3 — original blocker)
-    - SDK source: ``/tmp/openai-agents/src/agents/sandbox/sandboxes/docker.py:1434-1477``
+Pinned to ``openai-agents==0.14.6``. Bumping the SDK requires
+re-merging the parent body. Track upstream for an injection hook.
 """
 
 from __future__ import annotations