Expose MCP prompts that mirror skill workflows using tool names

[mglaman]

Context

The skills/drupalorg-cli/SKILL.md and reference files in skills/drupalorg-cli/references/ document end-to-end workflows using CLI command names (e.g. issue:apply, issue:branch). When surfaced as MCP prompts as-is, they're not useful to an MCP client because the MCP server exposes tools (e.g. get_issue, get_issue_branch_name), not CLI commands.

Proposed Work

Create PromptRegistry.php in src/Api/Mcp/ with #[McpPrompt]-annotated methods that provide workflow guidance written in terms of MCP tool names rather than CLI commands.

This could be implemented in a few ways:

1. Author new prompt content — write MCP-native workflow documentation that references tool names directly.
2. Transform skill content at runtime — post-process the existing skill markdown, substituting CLI command references with their MCP tool equivalents.
3. Separate prompt source files — add MCP-specific workflow markdown files alongside the existing skill references.

Acceptance Criteria

• listPrompts returns prompts for the main skill reference and each workflow (work-on-issue, patch contribution, GitLab MR contribution).
• Prompt content references MCP tool names (e.g. get_issue, list_merge_requests) rather than CLI commands.
• PHPStan level 6, PSR-2, and all unit tests continue to pass.