Add isolated Code Mode and reliable terminal execution

[fazxes]

Summary

• Add a restricted code tool for composing file reads, searches, stored-result reads, and foreground terminal commands in JavaScript.
• Keep nested operations on the existing validation, permission, execution, and lifecycle paths.
• Stop Code Mode before later effects after approval-required or indeterminate nested results.
• Run generated code in an isolated companion with no direct filesystem, network, environment, package, or subprocess access.
• Preserve final PTY output until the terminal host journals it before session completion.
• Normalize one stringified nested terminal request only when the full terminal schema is active.
• Show Code Mode activity without double-counting nested operations.
• Package, sign, and update the Code Mode companion alongside fx.

[vercel-security-reviewer]

Security review: https://reviewer.vercel-security.com/vercel-labs/fx/pull/540