fix: improve Markdown formatting instructions for system prompts

[tamuratak]

fix: improve Markdown formatting instructions for system prompts. Fix microsoft/vscode#283488

The new prompt copied from

vscode-copilot-chat/src/extension/prompts/node/agent/openai/gpt51CodexPrompt.tsx

Lines 110 to 116 in fcf50df

	<Tag name='special_formatting'>
	Use proper Markdown formatting:
	- Wrap symbol names (classes, methods, variables) in backticks: `MyClass`, `handleClick()`<br />
	- When mentioning files or line numbers, always follow the rules in fileLinkification section below:
	<FileLinkificationInstructions />
	<MathIntegrationRules />
	</Tag>

[Copilot]

Pull request overview

This PR improves Markdown formatting instructions in system prompts by clarifying the distinction between symbol names (which should use backticks) and file references (which should use markdown links). The changes align three GPT-5 family prompts with the pattern already established in gpt51CodexPrompt.tsx, addressing issue #283488.

Key Changes

• Removes ambiguous instruction to wrap filenames in backticks, which conflicted with the FileLinkificationInstructions component that explicitly forbids backticks for file references
• Adds explicit guidance that symbol names (classes, methods, variables) should use backticks while file/line references must follow FileLinkificationInstructions rules
• Consolidates formatting instructions in gpt51Prompt.tsx by moving FileLinkificationInstructions into the special_formatting section

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/extension/prompts/node/agent/openai/gpt5Prompt.tsx	Removes conflicting instruction to wrap filenames in backticks from reminder instructions
src/extension/prompts/node/agent/openai/gpt5CodexPrompt.tsx	Removes partial conflicting instruction and keeps only the general "Use proper Markdown formatting" guidance
src/extension/prompts/node/agent/openai/gpt51Prompt.tsx	Adds comprehensive special_formatting section clarifying symbol names use backticks while files follow FileLinkificationInstructions; moves FileLinkificationInstructions from final_answer_formatting to special_formatting for better organization

The changes look good and properly address the conflict between the previous instructions. The new pattern clearly separates concerns: backticks are for code symbols, and markdown links are for file references per the FileLinkificationInstructions component which explicitly states "NO BACKTICKS ANYWHERE" for file references.