fix: parse streamed command output fences

[duyua9]

Summary

• add a small parser helper that extracts the command from streamed completions without leaking markdown fences
• handle fenced commands with or without a language tag, split opening fences, inline fenced commands, and raw one-line commands
• add focused parser regression checks covering the known empty-script / literal-backticks issue shapes

Testing

• /Users/ming/code/BuilderIO/ai-shell/node_modules/.bin/prettier --write src/helpers/completion.ts src/helpers/completion.test.ts
• /Users/ming/code/BuilderIO/ai-shell/node_modules/.bin/jiti src/helpers/completion.test.ts
• /Users/ming/code/BuilderIO/ai-shell/node_modules/.bin/pkgroll

Closes #105
Closes #145
Closes #104