feat: add experimental gitoxide backend for git operations

[avihut]

Summary

• Adds gitoxide (gix crate) as an alternative git backend, controlled by the runtime config flag daft.experimental.gitoxide
• When enabled, daft uses gitoxide's native Rust library for ~20 supported read-only operations instead of spawning git subprocesses
• Old subprocess code stays intact and remains the default behavior
• Gracefully falls back to subprocess for operations gitoxide can't handle (worktree management, push, pull, stash, checkout, etc.)

Architecture

• GitCommand struct gains use_gitoxide: bool field and gix_repo: OnceLock<ThreadSafeRepository> for lazy repo discovery
• Builder pattern (.with_gitoxide(enabled)) avoids touching existing call sites
• New src/git_oxide.rs module (~700 lines) contains all gitoxide implementations
• Conditional dispatch at the top of each migrated method: if self.use_gitoxide { return git_oxide::...; }
• Runtime notice printed once via std::sync::Once: [experimental] Using gitoxide backend for git operations

Operations migrated

• Repository discovery & state: rev_parse_git_common_dir, is_inside_git_repo, is_inside_work_tree, is_bare_repository, get_git_dir, get_current_worktree_path
• References & branches: symbolic_ref_short_head, show_ref_exists, for_each_ref, branch_list_verbose
• Config reading: config_get, config_get_global
• Status & commit info: has_uncommitted_changes, rev_list_count
• Remote info: remote_list, remote_get_url
• Remote network: ls_remote_symref, ls_remote_heads, ls_remote_branch_exists

Bug fixes included

• Prune worktree deletion with CWD inside: gix::discover(".") stored relative paths, causing workdir() to return "." instead of an absolute path. Fixed by passing current_dir() to discover().
• Flow-eject on bare-repo layout: The cached gix repo discovered from the bare project root had no workdir, causing repo.status() to fail. Fixed by falling back to subprocess when the cached repo is bare.

Turning on the feature

git config --global daft.experimental.gitoxide true

Test plan

• cargo fmt -- --check passes
• cargo clippy -- -D warnings passes
• cargo test --lib — 183 passed, 0 failed
• just test-integration — default mode (gitoxide off)
• just test-integration with daft.experimental.gitoxide=true
• Manual smoke test: enable flag globally, run git worktree-prune, git worktree-flow-eject

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