fix: replace sync file read with async fs/promises and add error handling

[comzzy-comzzy]

Summary

Fixes #307 — Script crashes or hangs when reading the core TOML taxonomy file unsafely.

Changes

• Replaced the dummy index.js with a full async implementation using fs/promises
• Reads crates/core/src/taxonomy/data/contract.toml asynchronously (non-blocking)
• Explicitly specifies utf-8 encoding to prevent raw Buffer returns
• Added targeted try/catch with handling for specific Node.js error codes:
  • ENOENT — file not found: outputs actionable message about initializing the crates/core submodule
  • EACCES — permission denied: warns about file permission boundaries
  • EBUSY / EPERM — file locked or operation not permitted
  • Generic fallback for unexpected errors
• Parses the TOML content using @iarna/toml and outputs the taxonomy as formatted JSON
• All error messages are clean CLI-friendly diagnostics, no raw V8 stack traces

Testing

• Verified the script runs without blocking the event loop
• Tested ENOENT scenario (missing file) — outputs contextual guidance
• Tested EACCES scenario (restricted permissions) — outputs permission warning
• Tested happy path — successfully reads and parses the TOML taxonomy

Closes #307