fix(js): use single interpreter instance in AI adapters

[chaliy]

Summary

• All three AI adapters (ai.ts, anthropic.ts, openai.ts) created two separate interpreter instances (BashTool for metadata + Bash for execution), causing state divergence where files written to one instance were invisible to the other
• Replaced dual instances with a single BashTool instance that handles both metadata (systemPrompt, description) and execution (execute, writeFile)
• Added 9 tests verifying shared state between the exposed bash handle and tool execution across all three adapters

Test plan

• 9 new tests in __test__/ai-adapters.spec.ts verify VFS state is shared:
  • Files from files option readable via execute/handler
  • Files written via bash.writeFile visible in execute/handler
  • Files created via execute/handler readable via bash.readFile
• Full JS test suite (231 tests) passes

Closes #990