refactor(agents): relocate boundary-only infra out of shared/

Neither module is imported by any sibling agent package, so neither belongs in
the cross-agent shared kernel:

- checkpointer.py -> app/agents/runtime/checkpointer.py
  LangGraph Postgres checkpoint saver. It's cross-agent *runtime infra* wired by
  the boundary (app lifespan + anonymous_chat & multi_agent_chat flows), not
  agent code. New app/agents/runtime/ layer holds boundary-wired agent infra.

- shared/system_prompt.py + shared/prompts/ -> app/prompts/
  The legacy single-agent prompt composer. The live agents don't use it
  (main_agent has its own system_prompt/ builder; anonymous_chat builds inline);
  its only consumer is new_llm_config_routes for displaying default instructions.
  Moved to the existing non-agent prompt domain:
    system_prompt.py        -> app/prompts/default_system_instructions.py
    prompts/                 -> app/prompts/system_prompt_composer/

app/agents/shared/ now contains only genuinely cross-agent code: context,
middleware/{compaction,retry_after,dedup_tool_calls}, tools/.

NOTE: get_default_system_instructions() (LLM-config UI) composes from the legacy
library, which differs from what the live agents actually run -- pre-existing
latent staleness, not changed here.

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

@@ -6,12 +6,12 @@ from datetime import UTC, datetime
 
 import pytest
 
-from app.agents.shared.prompts.composer import (
+from app.db import ChatVisibility
+from app.prompts.system_prompt_composer.composer import (
     ALL_TOOL_NAMES_ORDERED,
     compose_system_prompt,
     detect_provider_variant,
 )
-from app.db import ChatVisibility
 
 pytestmark = pytest.mark.unit
 
@@ -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.shared.prompts.composer import detect_provider_variant
+        from app.prompts.system_prompt_composer.composer import detect_provider_variant
 
         assert detect_provider_variant("openai:gpt-5-codex") == "openai_codex"
         assert detect_provider_variant("openai:gpt-5") == "openai_reasoning"