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/agents/multi_agent_chat/main_agent/runtime/agent_cache.py

@@ -10,7 +10,7 @@ from langchain_core.language_models import BaseChatModel
 from langchain_core.tools import BaseTool
 from langgraph.types import Checkpointer
 
-from app.agents.new_chat.agent_cache import (
+from app.agents.shared.agent_cache import (
     flags_signature,
     get_cache,
     stable_hash,

surfsense_backend/app/agents/multi_agent_chat/main_agent/runtime/factory.py

@@ -19,7 +19,7 @@ from app.agents.multi_agent_chat.subagents import (
 from app.agents.multi_agent_chat.subagents.mcp_tools.index import (
     load_mcp_tools_by_connector,
 )
-from app.agents.new_chat.connector_searchable_types import (
+from app.agents.shared.connector_searchable_types import (
     map_connectors_to_searchable_types,
 )
 from app.agents.shared.feature_flags import AgentFeatureFlags, get_flags