[Feature]: Make default agent prompts task-agnostic outside coding workflows #8

Problem or opportunity

Diffex's default system prompt currently presents the assistant as an expert coding assistant operating inside Diffex.
That framing is appropriate for repository work, but it can over-steer sessions toward code reading, code editing, and software-specific workflows when the user wants a broader agentic task.
Users should be able to use Diffex for non-coding work such as document drafting, research synthesis, planning, data cleanup, project operations, and other tool-assisted tasks without replacing the entire default prompt.

Affected package: packages/coding-agent.
Possible shared boundary: packages/agent if prompt assembly or harness-level identity becomes more generic.

Desired outcome

The default prompt should describe Diffex as a general task execution agent/harness first, with coding as one supported workflow rather than the primary identity.
Coding-specific guidance should still be available when the selected tools, working directory, project context, or user request indicates software development work.

Success criteria

• A non-coding user request is not implicitly reframed as a coding task by the default system prompt.
• The default prompt still gives strong guidance for coding workflows when the user asks for repository, code, shell, or file-editing work.
• Tool and file safety guidance remains explicit and robust.
• Existing customization paths such as --system-prompt, --append-system-prompt, prompt files, skills, and extensions continue to work.
• Tests or prompt snapshots cover both coding and non-coding prompt behavior.

Affected area

Multiple areas

Proposed approach

Refactor the default prompt in packages/coding-agent/src/core/system-prompt.ts so the identity and behavior are task-agnostic.
Move coding-specific wording into conditional guidance, tool descriptions, or a coding workflow section that applies when relevant.
Keep the current high-quality engineering and file operation rules for coding tasks, but avoid making them the assistant's global purpose.

Alternatives considered

Relying on users to pass --system-prompt works for advanced users, but makes the default experience narrower than the harness capabilities.
Keeping the current prompt and adding non-coding examples may help, but the opening identity would still bias model behavior toward software work.

Additional context

The CLI help text also describes the product as an AI coding assistant, so the wording may need to be reviewed alongside prompt behavior.

Pre-submission checks

• I searched existing issues and did not find a duplicate request.
• I described the need and desired outcome, not only a preferred implementation.
• I did not include secrets, credentials, or private deployment details.