fix(git): use stdin for agent commit prompt and safe UTF-8 truncation

[ZhiAo1120]

Summary

• pipe commit prompts through stdin for Codex and Claude Code to avoid command-line length limits
• truncate staged diffs at valid UTF-8 boundaries before invoking the agent
• keep the change self-contained so it does not depend on the timeout configuration PR

Verification

• pnpm build
• cargo check --manifest-path src-tauri/Cargo.toml --quiet