refactor: Build agent prompts dynamically

Similar to the way the system prompt is built, the system prompts for
the agents are now built dynamically as well and use the same tool
descriptions.

* mevedel-agents.el (mevedel-agents--agents)
(mevedel-codebase-analyst-tools): Add defcustoms which define the tools
used for each agent.
Add defvars which declare the base system prompt.

* mevedel.el (mevedel-agents, mevedel-presets, mevedel-tools)
(mevedel-system): Reorder load order.

Author: FrauH0lle