feat(repo): implement is_git_repo via gix::discover (M1 R1) (#3)

WHAT: src/repo/is_git_repo.rs — `pub fn is_git_repo(path: &Path) -> bool` backed
by `gix::discover` (true iff `path` is inside a git working tree, including from
a subdirectory), with parity tests vs `git rev-parse --git-dir` on repo / subdir
/ non-repo fixtures. Registered in repo/mod.rs; the PyO3 `is_git_repo` wrapper is
wired to call it.

WHY: First read primitive of the M1 git-tend port (docs/ROADMAP.md M1, step 1).

PROVENANCE: the implementation and its tests were written by the local model
qwen2.5-coder:32b, driven headlessly through `newt worker` (the newt-agent ACP
worker) by the pilot. The pilot applied a mechanical clippy fixup
(needless_borrows_for_generic_args) and the module/PyO3 wiring.


Model: qwen2.5-coder:32b
Piloted-by: newt-agent

Co-authored-by: Shawn Hartsock <[email protected]>
Co-authored-by: qwen2.5-coder:32b <[email protected]>
Co-authored-by: Claude Opus 4.8 (1M context) <[email protected]>

Author: 3 people

src/python.rs

@@ -47,8 +47,8 @@ pub struct RepoStatus {
 // with a call into it. The pure-Rust core is what carries the gix logic + tests.
 
 #[pyfunction]
-fn is_git_repo(_path: String) -> PyResult<bool> {
-    todo!("repo::is_git_repo (gix::discover)")
+fn is_git_repo(path: String) -> PyResult<bool> {
+    Ok(crate::repo::is_git_repo(std::path::Path::new(&path)))
 }
 
 #[pyfunction]

src/repo/is_git_repo.rs

@@ -0,0 +1,86 @@
+use gix::discover;
+use std::path::Path;
+
+/// Returns true iff `path` is inside a git working tree (mirrors
+/// `git rev-parse --git-dir` exit==0).
+pub fn is_git_repo(path: &Path) -> bool {
+    discover(path).is_ok()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::repo::fixtures;
+    use std::process::Command;
+
+    #[test]
+    fn repo_root_and_subdir() {
+        let td = fixtures::repo();
+        let root_path = td.path();
+
+        // Test at the repo root
+        assert!(is_git_repo(root_path));
+
+        // Create a subdirectory and test there
+        let subdir_path = root_path.join("subdir");
+        std::fs::create_dir(&subdir_path).expect("mkdir");
+        assert!(is_git_repo(&subdir_path));
+    }
+
+    #[test]
+    fn non_repo() {
+        let td = tempfile::tempdir().expect("tempdir");
+        let non_repo_path = td.path();
+        assert!(!is_git_repo(non_repo_path));
+    }
+
+    #[test]
+    fn parity_with_git_cli() {
+        let repo_td = fixtures::repo();
+        let repo_path = repo_td.path();
+
+        // Check the repo path
+        assert_eq!(
+            is_git_repo(repo_path),
+            Command::new("git")
+                .args(["-C", &repo_path.to_string_lossy(), "rev-parse", "--git-dir"])
+                .status()
+                .expect("spawn git")
+                .success()
+        );
+
+        // Check a subdirectory within the repo
+        let subdir_path = repo_path.join("subdir");
+        std::fs::create_dir(&subdir_path).expect("mkdir");
+        assert_eq!(
+            is_git_repo(&subdir_path),
+            Command::new("git")
+                .args([
+                    "-C",
+                    &subdir_path.to_string_lossy(),
+                    "rev-parse",
+                    "--git-dir"
+                ])
+                .status()
+                .expect("spawn git")
+                .success()
+        );
+
+        // Check a non-repo path
+        let non_repo_td = tempfile::tempdir().expect("tempdir");
+        let non_repo_path = non_repo_td.path();
+        assert_eq!(
+            is_git_repo(non_repo_path),
+            Command::new("git")
+                .args([
+                    "-C",
+                    &non_repo_path.to_string_lossy(),
+                    "rev-parse",
+                    "--git-dir"
+                ])
+                .status()
+                .expect("spawn git")
+                .success()
+        );
+    }
+}

src/repo/mod.rs

@@ -20,6 +20,9 @@ pub use crate::error::{GitxtendError, Result};
 // ---- method registrations (one block per implemented method) -------------
 // (methods land here as M1 progresses — see docs/ROADMAP.md M1 ordering)
 
+mod is_git_repo;
+pub use is_git_repo::is_git_repo;
+
 /// Temp-dir git fixtures shared by the per-method parity tests.
 ///
 /// Fixtures are built with the real `git` CLI, so each parity test asserts