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/runtime/session_manager.py

@@ -144,13 +144,3 @@ async def cleanup(scan_id: str) -> None:
             "cleanup(%s): client.delete raised; container may need manual reaping",
             scan_id,
         )
-
-
-def cached_scan_ids() -> list[str]:
-    """Snapshot of currently-cached scan ids. Used by the TUI / CLI."""
-    return list(_SESSION_CACHE.keys())
-
-
-def _reset_cache_for_tests() -> None:
-    """Test helper — clears the module cache between unit tests."""
-    _SESSION_CACHE.clear()