docs: fix development setup issues and CLI build instructions

[chriscool]

   - Clarify Node.js version requirement as exactly v20.19.2 in DEVELOPMENT.md
   - Add notes that devcontainer and Nix environments provide correct Node version
   - Add troubleshooting for Git LFS hook conflicts and Node version issues
   - Fix CLI build script name typo (vsix:unpackaged -> vsix:unpacked)
   - Correct CLI build order: move deps:install after build step
   - Add missing .env file requirement for CLI in dist directory

These changes address common setup issues encountered when building KiloCode from source, including Git LFS conflicts, Node version mismatches, and incorrect CLI build sequences that were causing build failures."

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