chore: replace deprecated sg command with ast-grep

[SynthLuvr]

Summary

Replace all four pnpm exec sg scan ... invocations in package.json
with pnpm exec ast-grep scan ...:

• strip-braces
• lint:exports
• lint:functions
• lint:file-comment

The sg CLI binary (shipped by @ast-grep/cli) is deprecated as of
v0.45.0 and prints a warning when run:

========================================================================
WARNING: `sg` is deprecated. Use `ast-grep` instead.
========================================================================

This project is currently on @ast-grep/cli@^0.44.1, so the warning
does not fire yet. However, the ast-grep binary is already available
and is a drop-in replacement, so this change is forward-compatible — it
keeps the project aligned with the upstream template and avoids the
deprecation warning once the dependency is bumped to >=0.45.0.

Backport of SynthLuvr/typescript-template#18.

Verification

pnpm build   # exit 0
pnpm lint    # exit 0 (biome, oxlint, ast-grep scans, md, peer-deps)
pnpm test    # all passed

All three pass with zero errors.