fix: make mcp config writes idempotent

Summary

This PR makes dev3000’s automatic MCP config updates idempotent and safer when config files already contain a dev3000 entry.

• Add a shared upsertMcpServerConfig helper for Claude, Cursor, and OpenCode config shapes
• Update stale dev3000 localhost MCP URLs when the MCP port changes
• Avoid rewriting config files when the dev3000 entry is already up to date
• Preserve existing MCP servers and top-level config fields
• Preserve OpenCode enabled: false
• Avoid overwriting custom user-managed dev3000 entries

Why

dev3000 keeps MCP client config files around between runs. If the MCP port changes, an existing dev3000 entry can point at a stale localhost URL, while repeated runs should not rewrite files that are already current.

This keeps the auto-config behavior useful while making the write path more conservative and predictable.

Tests

• pnpm.cmd test src\utils\mcp-configs.test.ts
• pnpm.cmd exec tsc --noEmit --pretty false
• pnpm.cmd exec biome check src\dev-environment.ts src\utils\mcp-configs.ts src\utils\mcp-configs.test.ts
• git diff --check