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/shared/system_prompt.py

@@ -0,0 +1,133 @@
+"""
+Thin compatibility wrapper around :mod:`app.agents.shared.prompts.composer`.
+
+The composer split the previous monolithic prompt string into a fragment
+tree under ``prompts/`` plus a model-family dispatch step (see the
+composer module docstring for credits). This module preserves the public
+function surface (``build_surfsense_system_prompt`` /
+``build_configurable_system_prompt`` /
+``get_default_system_instructions`` / ``SURFSENSE_SYSTEM_PROMPT``) so
+that existing call sites — `chat_deepagent.py`, anonymous chat routes,
+and the configurable-prompt admin path — keep working without churn.
+
+For new call sites prefer importing ``compose_system_prompt`` directly
+from :mod:`app.agents.shared.prompts.composer`.
+"""
+
+from __future__ import annotations
+
+from datetime import UTC, datetime
+
+from app.db import ChatVisibility
+
+from .prompts.composer import (
+    _read_fragment,
+    compose_system_prompt,
+    detect_provider_variant,
+)
+
+# Optional routing fragments under ``prompts/routing/`` (see composer).
+_DEFAULT_CONNECTOR_ROUTING: tuple[str, ...] = ("linear", "slack")
+
+# Public re-exports for backwards compatibility (some legacy code reads the
+# raw default-instructions text directly).
+SURFSENSE_SYSTEM_INSTRUCTIONS_TEMPLATE = (
+    "<system_instruction>\nDefault SurfSense agent system instructions are now\n"
+    "composed from prompts/base/*.md. See compose_system_prompt() for details.\n"
+    "</system_instruction>"
+)
+
+# Citation block re-exposed for legacy importers that referenced this constant
+# directly. The composer is the canonical source; this is a frozen snapshot
+# loaded at module-init time.
+SURFSENSE_CITATION_INSTRUCTIONS = _read_fragment("base/citations_on.md")
+SURFSENSE_NO_CITATION_INSTRUCTIONS = _read_fragment("base/citations_off.md")
+
+
+def build_surfsense_system_prompt(
+    today: datetime | None = None,
+    thread_visibility: ChatVisibility | None = None,
+    enabled_tool_names: set[str] | None = None,
+    disabled_tool_names: set[str] | None = None,
+    mcp_connector_tools: dict[str, list[str]] | None = None,
+    *,
+    model_name: str | None = None,
+) -> str:
+    """Build the default SurfSense system prompt (citations on, defaults).
+
+    See :func:`app.agents.shared.prompts.composer.compose_system_prompt`
+    for full parameter docs.
+    """
+    return compose_system_prompt(
+        today=today,
+        thread_visibility=thread_visibility,
+        enabled_tool_names=enabled_tool_names,
+        disabled_tool_names=disabled_tool_names,
+        mcp_connector_tools=mcp_connector_tools,
+        citations_enabled=True,
+        model_name=model_name,
+        connector_routing=_DEFAULT_CONNECTOR_ROUTING,
+    )
+
+
+def build_configurable_system_prompt(
+    custom_system_instructions: str | None = None,
+    use_default_system_instructions: bool = True,
+    citations_enabled: bool = True,
+    today: datetime | None = None,
+    thread_visibility: ChatVisibility | None = None,
+    enabled_tool_names: set[str] | None = None,
+    disabled_tool_names: set[str] | None = None,
+    mcp_connector_tools: dict[str, list[str]] | None = None,
+    *,
+    model_name: str | None = None,
+) -> str:
+    """Build a configurable SurfSense system prompt (NewLLMConfig path).
+
+    See :func:`app.agents.shared.prompts.composer.compose_system_prompt`
+    for full parameter docs.
+    """
+    return compose_system_prompt(
+        today=today,
+        thread_visibility=thread_visibility,
+        enabled_tool_names=enabled_tool_names,
+        disabled_tool_names=disabled_tool_names,
+        mcp_connector_tools=mcp_connector_tools,
+        custom_system_instructions=custom_system_instructions,
+        use_default_system_instructions=use_default_system_instructions,
+        citations_enabled=citations_enabled,
+        model_name=model_name,
+        connector_routing=_DEFAULT_CONNECTOR_ROUTING,
+    )
+
+
+def get_default_system_instructions() -> str:
+    """Return the default ``<system_instruction>`` block (no tools / citations).
+
+    Useful for populating the UI when seeding ``NewLLMConfig.system_instructions``.
+    The output reflects the current fragment tree, not a baked-in constant.
+    """
+    resolved_today = datetime.now(UTC).date().isoformat()
+    from .prompts.composer import _build_system_instructions  # local import
+
+    return _build_system_instructions(
+        visibility=ChatVisibility.PRIVATE,
+        resolved_today=resolved_today,
+    ).strip()
+
+
+# Backwards compatibility — some modules import the constant directly.
+SURFSENSE_SYSTEM_PROMPT = build_surfsense_system_prompt()
+
+
+__all__ = [
+    "SURFSENSE_CITATION_INSTRUCTIONS",
+    "SURFSENSE_NO_CITATION_INSTRUCTIONS",
+    "SURFSENSE_SYSTEM_INSTRUCTIONS_TEMPLATE",
+    "SURFSENSE_SYSTEM_PROMPT",
+    "build_configurable_system_prompt",
+    "build_surfsense_system_prompt",
+    "compose_system_prompt",
+    "detect_provider_variant",
+    "get_default_system_instructions",
+]