feat: pass LANGWATCH_API_KEY to MCP server config

[rogeriochaves]

Summary

• Pass LANGWATCH_API_KEY via the env field in the LangWatch MCP server config, as required by the updated docs
• Gitignore .mcp.json (and .cursor/mcp.json) since it now contains API key secrets
• Generate a .mcp.json.example with placeholder values so the config shape is committed for team reference
• Update the initial prompt to instruct the coding agent to include .mcp.json.example copy instructions in the project README
• Replace single-line input for project goal with a custom raw-stdin textarea prompt that supports:
  • Shift+Enter for newlines (detects \n vs \r — works in Warp and other terminals)
  • Bracketed paste mode for pasting multiline text without submitting
  • Arrow key navigation, backspace across lines, Home/End, Delete
  • CSI u / kitty keyboard protocol support for Shift+Enter
• Reorder coding assistant choices: move Kilocode after Cursor

Test plan

• Run better-agents init and verify .mcp.json contains the real API key in env.LANGWATCH_API_KEY
• Verify .mcp.json.example is generated with "your_langwatch_api_key_here" placeholder
• Verify .gitignore includes .mcp.json and .cursor/mcp.json
• Verify the LangWatch MCP server connects successfully with the new config format
• Run init on a project with an existing .gitignore and confirm .mcp.json entry is appended
• Test textarea: type single line + Enter → submits
• Test textarea: Shift+Enter → adds newline, continues editing
• Test textarea: paste multiline text → all lines appear, no premature submit
• Test textarea: backspace on empty line → merges with previous line
• Test textarea: empty input + Enter → validation error shows
• Verify coding assistant list shows Kilocode after Cursor