Instruct LLM to use pull request template when creating PRs

[almaleksia]

Adds server instructions that tell LLM to search for PR templates and use them for creating PRs.

xref #1076

[Copilot]

Pull Request Overview

This PR enhances the GitHub MCP Server by instructing LLMs to search for and use pull request templates when creating PRs. The instruction is conditionally added only when both the "pull_requests" and "repos" toolsets are enabled, ensuring the LLM has access to the necessary file retrieval tools.

Key Changes

• Modified getToolsetInstructions() to accept an enabledToolsets parameter for conditional instruction generation
• Added new instruction text for pull request template discovery and usage when repos toolset is available
• Enhanced test coverage with assertions to verify the conditional template instruction behavior

Reviewed Changes

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

File	Description
pkg/github/instructions.go	Updated function signature to accept enabled toolsets; added conditional logic to include PR template instructions when repos toolset is available
pkg/github/instructions_test.go	Added strings import; expanded test structure with fields for conditional assertions; added test cases to verify template instructions appear/disappear based on enabled toolsets

Comments suppressed due to low confidence (1)

pkg/github/instructions_test.go:161

• The test cases for "issues", "discussions", and "nonexistent" toolsets are missing the enabledToolsets field. Since getToolsetInstructions now requires this parameter, these test cases should provide it (even if it's just nil or an empty slice []string{}) to be explicit about the expected behavior and avoid potential issues if the function logic changes to use this parameter for these toolsets.

		{
			toolset:       "issues",
			expectedEmpty: false,
		},
		{
			toolset:       "discussions",
			expectedEmpty: false,
		},
		{
			toolset:       "nonexistent",
			expectedEmpty: true,
		},