Prompts: rework get_prompt() to read from EngineConfig (mirror MCP registry pattern)

[ahmedennaifer]

Problem

idun_agent_engine.prompts.helpers.get_prompt() resolves through YAML file → Manager API only. The DB-backed EngineConfig.prompts assembled by idun_agent_standalone.services.engine_config.assemble_engine_config() on every boot and reload is never consulted by user agent code that calls get_prompt("system_prompt").

Symptoms:

• Editing a prompt via /admin/api/v1/prompts mutates the DB, but user code keeps reading YAML values.
• User code that calls get_prompt(...) at module import time snapshots the YAML value forever — even /reload doesn't update it.
• The constitution says "DB is steady-state truth in standalone. YAML seeds at first boot only." Today's helper violates that for prompts.

Direction

Follow the MCP registry pattern that already exists in the codebase. See libs/idun_agent_engine/src/idun_agent_engine/mcp/registry.py:

```python
_active_registry: MCPClientRegistry | None = None

def set_active_registry(registry: MCPClientRegistry | None) -> None: ...
def get_active_registry() -> MCPClientRegistry | None: ...
```

Mirror for prompts:

```python

libs/idun_agent_engine/src/idun_agent_engine/prompts/registry.py

_active_prompts: list[PromptConfig] | None = None

def set_active_prompts(prompts: list[PromptConfig] | None) -> None: ...
def get_active_prompts() -> list[PromptConfig] | None: ...
```

Wire-up:

• Boot — libs/idun_agent_standalone/src/idun_agent_standalone/app.py: after assemble_engine_config(session) returns, call set_active_prompts(engine_config.prompts).
• Reload — libs/idun_agent_standalone/src/idun_agent_standalone/services/reload.py:commit_with_reload: in the success branch after reload_callable(assembled) returns, call set_active_prompts(assembled.prompts). Never in the failure branch (rollback preserves the prior prompts).

Resolution order in get_prompt() (and get_prompts()):

1. Explicit config_path= arg (tests, scripts).
2. get_active_prompts() if not None — standalone path (new).
3. IDUN_CONFIG_PATH YAML (bare engine + YAML, unchanged).
4. Manager API (SaaS / Manager path, unchanged).

This keeps all three deployment shapes working, adds the in-process snapshot for standalone, and matches the MCP precedent so the next contributor reads it as a familiar pattern rather than a new abstraction.

Acceptance criteria

• prompts/registry.py ships with set_active_prompts + get_active_prompts, mirroring mcp/registry.py shape.
• get_prompt() resolution order updated; YAML/Manager paths untouched for non-standalone deployments.
• Standalone boot writes set_active_prompts(engine_config.prompts) after assembly.
• Standalone commit_with_reload writes set_active_prompts(assembled.prompts) in the success branch only.
• Unit test: standalone test boots, edits a prompt via admin REST, calls get_prompt(...) and observes the new value (no `/reload` needed beyond the normal admin pipeline).
• Unit test: bare-engine YAML mode still resolves through IDUN_CONFIG_PATH.
• Unit test: SaaS Manager mode still resolves through get_prompts_from_api().
• Autouse test fixture clears _active_prompts between tests so state doesn't bleed.

Out of scope

• Prompt versioning / labels (Langfuse-style `label='production'` semantics). Track separately if wanted.
• HTTP self-call to /admin/api/v1/prompts — explicitly rejected, see design discussion.
• DB-direct access from a sync sessionmaker in standalone — rejected in favor of in-memory snapshot.