[Policy] git_discovery 使用 git 命令而非 libgit2

[TrueNine]

🐛 问题描述

sdk/src/policy/git_discovery.rs 调用外部 git 命令而非 libgit2 库，增加依赖且跨平台处理复杂。

📁 证据

git_discovery.rs

✅ 修复建议

考虑使用 git2 crate 替代 shell out。

[TrueNine]

Fixed in ef04b48.

Change: Replaced manual filesystem-based git detection (fs::symlink_metadata, fs::read_to_string, gitdir: parsing) with the git2 crate (libgit2 bindings).

• resolve_git_info_dir: uses git2::Repository::open() instead of manually checking for .git dir/file
• find_all_git_repos: uses git2::Repository::open() as the sole detection mechanism (bare repos, linked worktrees, gitlinked submodules all supported)
• .git added to SKIP_DIRS to prevent walking into .git/ directories
• Tests updated to use git2::Repository::init() for creating real repos
• Build: sdk/.cargo/config.toml added with CMAKE path for vendored libgit2

Old code fully removed — no compatibility fallback retained.

Test: 381 passed, 0 failed.

Implements #345.