chore: replace deprecated sg command with ast-grep

[SynthLuvr]

Summary

The sg CLI binary (shipped by @ast-grep/cli) is deprecated as of v0.45.0. Running it now prints a deprecation warning:

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

This replaces 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

Investigation

• Confirmed sg and ast-grep are both binaries provided by @ast-grep/cli (already at ^0.45.0 in package.json/lockfile).
• Running pnpm exec sg --version against v0.45.0 emits the deprecation warning shown above, whereas ast-grep does not.
• Verified sg is only referenced as a command in these four scripts (no sgconfig.yml or docs references).

Verification

pnpm build   # exit 0 (tsc)
pnpm lint    # exit 0 (biome, oxlint, ast-grep scans, md, peer-deps)
pnpm test    # 3 passed

All three pass with zero errors, and the deprecation warning is gone.