fix(chat): render builtin command output safely

[devswha]

Summary

• normalize SDK builtin stdout with Node’s maintained VT-control stripper
• emit builtin output as tagged completed plain-text messages so realtime state preserves provenance
• render tagged output as escaped preformatted text instead of Markdown/KaTeX
• preserve trusted requested export paths when upstream diagnostics contain corrupted replacement characters

Verification

• focused Bun SDK contract: 1 passed
• MessageComponent regressions: 3 passed
• ESLint on all changed files
• full client/server typecheck

Closes #5