fix(init): skip binary automatic pre-reads - #1402

The init preflight eagerly reads a small allowlist of common manifests and config files into initial workflow state. A binary file masquerading under one of those names should not be decoded and injected before any agent has asked for it.

Summary

• Read automatic preflight candidates as bytes and apply the existing bounded NUL-byte classifier before UTF-8 decoding
• Store the existing null sentinel when an automatic pre-read candidate is binary
• Leave the full directory listing unchanged, including binary paths
• Deliberately leave the generic read-files tool unchanged: after an agent selects a path from the listing, the existing maxBytes-bounded read is allowed
• Add a regression test proving that automatic binary pre-reads are skipped while an explicitly requested binary path remains readable

The follow-up #1404 adds scan and read diagnostics without changing that distinction.

Test plan

• 21 focused pre-read and filesystem-tool tests
• Biome check on changed files
• pnpm typecheck