feat: add texlab language server and latexindent formatter

[edlsh]

Summary

Add LaTeX LSP support via TexLab language server.

Changes

• Add TexLab LSP server with auto-download from GitHub releases
• Add latexindent formatter for .tex files

Features

• Code completion for commands, environments, citations, references
• Hover documentation
• Go-to-definition for labels and citations
• Diagnostics via chktex (when installed)
• BibTeX support (.bib files)

Files Modified

• packages/opencode/src/lsp/server.ts - Added TexLab LSP server
• packages/opencode/src/format/formatter.ts - Added latexindent formatter

Tests Run

• bun run typecheck - Passed (opencode package)
• bun test - 255 passed, 1 skipped, 0 failed
• bun dev --help - Verified code compiles and runs
• Verified asset naming against actual texlab GitHub releases (confirmed correct):
  • texlab-aarch64-macos.tar.gz
  • texlab-x86_64-macos.tar.gz
  • texlab-x86_64-linux.tar.gz
  • texlab-x86_64-windows.zip

[rekram1-node]

/review

[github-actions[bot]]

lgtm