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/tests/unit/agents/new_chat/prompts/test_composer.py

@@ -6,7 +6,7 @@ from datetime import UTC, datetime
 
 import pytest
 
-from app.agents.new_chat.prompts.composer import (
+from app.agents.shared.prompts.composer import (
     ALL_TOOL_NAMES_ORDERED,
     compose_system_prompt,
     detect_provider_variant,
@@ -64,7 +64,7 @@ class TestProviderVariantDetection:
         ``gpt-5`` reasoning regex first. Codex is the more specialised
         prompt and mirrors OpenCode's dispatch order.
         """
-        from app.agents.new_chat.prompts.composer import detect_provider_variant
+        from app.agents.shared.prompts.composer import detect_provider_variant
 
         assert detect_provider_variant("openai:gpt-5-codex") == "openai_codex"
         assert detect_provider_variant("openai:gpt-5") == "openai_reasoning"

surfsense_backend/tests/unit/agents/new_chat/test_agent_cache.py

@@ -16,7 +16,7 @@ from dataclasses import dataclass
 
 import pytest
 
-from app.agents.new_chat.agent_cache import (
+from app.agents.shared.agent_cache import (
     flags_signature,
     reload_for_tests,
     stable_hash,