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/tools/thinking/__init__.py

@@ -1,4 +0,0 @@
-from .tool import think
-
-
-__all__ = ["think"]

strix/tools/thinking/tool.py

@@ -4,10 +4,10 @@ from __future__ import annotations
 
 import json
 
-from strix.tools._decorator import strix_tool
+from agents import function_tool
 
 
-@strix_tool(timeout=10)
+@function_tool(timeout=10)
 async def think(thought: str) -> str:
     """Record a private chain-of-thought note. No side effects, no new info.