refactor: nuke `strix_tool` shim + dead package re-exports

``@strix_tool`` was passing through every kwarg to ``@function_tool``
with the same defaults — zero Strix-specific value-add. The docstring
also still claimed terminal/browser/python tools opted into
``timeout_behavior="raise_exception"``, but those tools were all
deleted in the recent migrations.

- Replace 30 ``@strix_tool(...)`` callsites with ``@function_tool(...)``.
- Inline ``dump_tool_result(x)`` as ``json.dumps(x, ensure_ascii=False,
  default=str)`` at all 64 callsites — no helper.
- Delete ``strix/tools/_decorator.py``.

Drive-by: gut dead package re-exports.

- ``strix/{agents,orchestration,tools}/__init__.py`` re-exported
  symbols nobody imports via the package — every consumer uses deep
  paths (``from strix.agents.factory import build_strix_agent``).
- The 8 ``strix/tools/<sub>/__init__.py`` re-exports only fed the
  splat ``from .agents_graph import *`` etc. in the parent package
  init, which is also gone now.
- Reduced to docstrings (or empty) so ``import strix.tools`` doesn't
  drag every tool's transitive deps in eagerly.

Drive-by: drop dead helpers in ``runtime.session_manager``
(``cached_scan_ids``, ``_reset_cache_for_tests``) — zero callers since
``tests/`` was nuked in ``a6d578c``.

Verified all tool timeouts preserved (think=10, list_requests=120,
finish_scan=60, web_search=330) and ruff/mypy at baseline.

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

strix/agents/__init__.py

@@ -1,19 +1,12 @@
-"""Strix agent package.
+"""Strix agent assembly.
 
-Public surface:
+- :func:`strix.agents.factory.build_strix_agent` — assemble a root or
+  child ``SandboxAgent``.
+- :func:`strix.agents.factory.make_child_factory` — closure factory
+  passed via context to the multi-agent ``create_agent`` graph tool.
+- :func:`strix.agents.prompt.render_system_prompt` — render the Jinja
+  system prompt.
 
-- :func:`build_strix_agent` — assemble a root or child ``agents.Agent``.
-- :func:`make_child_factory` — closure factory passed via context to
-  the multi-agent ``create_agent`` graph tool.
-- :func:`render_system_prompt` — render the Jinja system prompt.
+Import deeply so ``import strix.agents`` doesn't pull every submodule's
+deps in eagerly.
 """
-
-from .factory import build_strix_agent, make_child_factory
-from .prompt import render_system_prompt
-
-
-__all__ = [
-    "build_strix_agent",
-    "make_child_factory",
-    "render_system_prompt",
-]