dev: initial setup for Claude Code development environment

[offich]

Description

Set up the initial development environment for Claude Code to improve the development experience, including CLAUDE.md, MCP configuration, Serena LSP, and Claude Skills.

What was done

• Added CLAUDE.md (project overview, commands, and architecture as instructions for Claude)
• Added .mcp.sample.json (sample configuration for GitHub MCP server)
• Added .mcp.json to .gitignore (exclude personal config file from VCS)
• Added CLAUDE.md to .pubignore (exclude from pub.dev publishing)
• Added .serena/ (Serena LSP project configuration for Dart)
• Added .claude/skills/create-pr/ (PR creation skill)
• Added .claude/skills/review-pr/ (PR review skill)
• Added docs/dev/review-guideline.md (review guidelines specific to DevTools Extensions)

What was NOT done

• No changes to existing application code (lib/)
• No changes to CI workflows
• No dependency updates

Operation Confirmation

Setup & Steps

1. Copy .mcp.sample.json to .mcp.json and set PAT to enable GitHub MCP
2. Verify that /create-pr and /review-pr slash commands work in Claude Code

Notes / Related URLs

N/A