Fix determinism checks for prompt mentions

[Michaelliv]

Summary

• replace the raw-source nondeterminism regex with an AST-aware check
• allow Date.now(), Math.random(), and new Date() mentions in prompts, metadata, comments, and string literals
• continue rejecting executable Date.now(), Math.random(), and new Date() usage, including optional chaining and static computed-property forms
• add a runtime regression for prompts that mention nondeterministic API names

Fixes #17

Verification

• npm test — 24 tests pass