refactor(agents): move connector_searchable_types, agent_cache, system_prompt + prompts to app/agents/shared (slice 7b)

Three live shared leaves discovered while taking stock after slice 7 (all are
consumed by the multi-agent stack and/or live routes, not single-agent-only):

- connector_searchable_types -> shared + shim (multi-agent factory uses it)
- agent_cache -> shared + shim (multi-agent runtime/agent_cache uses it)
- system_prompt + prompts/ (42 .md fragments) -> shared together + shim.
  Repointed composer's _PROMPTS_PACKAGE to app.agents.shared.prompts so
  importlib.resources fragment loading keeps working; system_prompt's relative
  ".prompts.composer" import is preserved by moving both as a unit.

Each keeps a re-export shim for the frozen chat_deepagent. After this slice,
new_chat/ holds only the frozen single-agent stack (chat_deepagent, subagents/,
__init__) plus shims.

surfsense_backend/app/routes/new_llm_config_routes.py

@@ -13,7 +13,7 @@ from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.future import select
 
-from app.agents.new_chat.system_prompt import get_default_system_instructions
+from app.agents.shared.system_prompt import get_default_system_instructions
 from app.config import config
 from app.db import (
     NewLLMConfig,