Remove Go-era cruft, update THIRD_PARTY.md for pure C project

- Delete cmd/codebase-memory-mcp/assets/ — skills and codex instructions
  are embedded as C string literals in src/cli/cli.c since v0.5.0
- Move BENCHMARK.md to docs/ to reduce root clutter
- Fix setup scripts: replace stale go build with scripts/build.sh
- THIRD_PARTY.md: replace Go module dependencies with vendored C
  libraries (sqlite3, mimalloc, mongoose, yyjson, xxhash, tre),
  fix CGo reference to static compilation