refactor(agents): colocate main_agent-only kernel into main_agent/

Move modules out of agents/shared/ that are consumed by a single package
(main_agent), placing each next to its only consumer instead of in a
"shared" grab-bag:

- agent_cache.py        -> main_agent/runtime/agent_cache_store.py
- connector_searchable_types.py -> main_agent/runtime/
- plugin_loader.py + plugins/    -> main_agent/plugins/
- skills/ + skills_backends.py   -> main_agent/skills/
- tools/invalid_tool.py          -> main_agent/tools/

Drop the skills_backends re-export from the shared middleware barrel and
repoint all consumers + tests. No behavior change; import-all,
error-contract, and the moved tests stay green.

surfsense_backend/app/agents/shared/tools/invalid_tool.py

@@ -1,53 +0,0 @@
-"""
-The ``invalid`` fallback tool.
-
-When the model emits a tool call whose name doesn't match any registered
-tool, :class:`ToolCallNameRepairMiddleware` rewrites the call to ``invalid``
-with the original name and a parser/validation error string. This tool's
-execution then returns that error to the model so it can self-correct.
-
-Ported from OpenCode's ``packages/opencode/src/tool/invalid.ts`` —
-LangChain has no equivalent fallback path; the default behavior on an
-unknown tool name is a hard ``ToolNotFoundError`` which kills the turn.
-
-Critically, the :class:`ToolDefinition` for this tool is **excluded** from
-the system-prompt tool list and from ``LLMToolSelectorMiddleware`` selection
-(see ``ToolDefinition.always_include`` filtering in the registry) — the
-model never advertises ``invalid`` as a callable. It only ever shows up
-in the tool registry so LangGraph can dispatch the rewritten call.
-"""
-
-from __future__ import annotations
-
-from langchain_core.tools import tool
-
-INVALID_TOOL_NAME = "invalid"
-INVALID_TOOL_DESCRIPTION = "Do not use"
-
-
-def _format_invalid_message(tool: str | None, error: str | None) -> str:
-    """Return the user-visible error string. Mirrors ``invalid.ts``."""
-    name = tool or "<unknown>"
-    detail = error or "(no error message provided)"
-    return (
-        f"The arguments provided to the tool `{name}` are invalid: {detail}\n"
-        f"Read the tool's docstring carefully and try again with valid arguments."
-    )
-
-
-@tool(name_or_callable=INVALID_TOOL_NAME, description=INVALID_TOOL_DESCRIPTION)
-def invalid_tool(tool: str | None = None, error: str | None = None) -> str:
-    """Return a human-readable explanation of a tool-call validation failure.
-
-    Activated only when :class:`ToolCallNameRepairMiddleware` rewrites a
-    failed tool call to ``invalid`` with the original tool name and the
-    error message produced during validation.
-    """
-    return _format_invalid_message(tool, error)
-
-
-__all__ = [
-    "INVALID_TOOL_DESCRIPTION",
-    "INVALID_TOOL_NAME",
-    "invalid_tool",
-]