Fix inconsistent JSON schema for explanation field in prompt template

[Copilot]

The promptTemplate JSON schema example omitted the "explanation" field, while explainInstruction (injected when --explain is active) required it. This gave the model conflicting instructions about the expected output shape.

Changes

• internal/llm/prompt.go: Added "explanation" as an optional field in the command object schema example; added a rule explicitly stating it is only required when requested
• internal/llm/prompt_test.go: Extended TestBuildSystemPrompt_ContainsJSONInstructions to assert "explanation" is present in the base schema

{
  "command": "the shell command to run",
  "description": "brief explanation of what it does",
  "risk": "safe or risky",
  "certainty": 90,
  "explanation": "(optional) step-by-step breakdown of the command's flags and arguments"
}

The rule list now includes: "explanation" is optional and only required when explicitly requested.

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[Copilot]

Pull request overview

This PR resolves a mismatch in the LLM system prompt instructions by ensuring the shell command JSON example includes the explanation field (marked optional) while explicitly stating it’s only required when --explain is active, aligning the base schema example with the injected explainInstruction.

Changes:

• Updated the command JSON example in promptTemplate to include an optional explanation field.
• Added an explicit rule stating explanation is optional unless explicitly requested.
• Extended the prompt test to assert explanation is present in the base JSON instructions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
internal/llm/prompt.go	Updates the system prompt’s command JSON example and rules to include/clarify explanation.
internal/llm/prompt_test.go	Ensures tests assert explanation is present in the base prompt JSON instructions.

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