refactor(prompt): simplify system.st to minimal agent identity

[carlosquijano]

Problem

system.st currently includes infrastructure details (runtime, I/O model, AI backend)
that are irrelevant to the agent's identity and will drift as the platform evolves.

Change

Strip the base prompt down to its essential purpose: identify the agent as an instance
of the minimal-java-agent open source template — provider-agnostic, infrastructure-agnostic,
ready to be extended via the extension prompt layer.

Acceptance Criteria

• No references to runtime, AI backend, or deployment details
• Communicates: open source template, extendable, agnostic
• README stays consistent