Rename Agent Instructions and add System Prompt

@marcus_johansson Could you please clarify a few details:

• Is the intention to update only the System Prompt field within ai_agents/src/Form/AiAgentForm.php, or are there other locations that need to be updated as well?
• Could you elaborate on the purpose and use case for introducing a separate "System Prompt" in addition to "Agent Instructions"?

@annmarysruthy, some clarifications why the difference:
A system prompt defines the overall role, goals, and constraints of an AI agent or model, acting as a foundational framework for its behavior.
Agent instructions, on the other hand, are more specific instructions or tasks given to an agent for a particular interaction or task. System prompts are persistent and apply across multiple interactions, while agent instructions are typically for a single interaction
Source:
https://blog.promptlayer.com/system-prompt-vs-user-prompt-a-comprehensiv...'s,%2C%20best%20practices%2C%20and%20more.

annmarysruthy - I'll edit the issue for a better explanation. I just saved it now because we saw it as a blocker.

@annmarysruthy - should this be in review?

Looks pretty good, had just one non-consequential question at https://git.drupalcode.org/project/ai_agents/-/merge_requests/102#note_5... .

@marcus_johansson No. There is a remaining part of solution: On the runner make sure to run the secured_system_prompt and replace the tokens and use this.

I could not fix this part. Therefore, Updated the issue description with remaining task and unassigned myself.

The token replacement is now added, as well as adding so current empty agents will set [agent_instructions] on next save.

Getting merged