feat(vcs): wire cache-vcs-dir setting through to drivers

DriverConfig used to expose a generic cache_dir hardcoded to a
relative ".cache/mozart/vcs", with each driver appending its own
"git"/"hg" subdir. As a result, the cache-vcs-dir setting (and
COMPOSER_CACHE_VCS_DIR env var) had no effect on where mirrors
were actually stored.

Replace cache_dir with cache_vcs_dir, resolving its default the
same way Composer does (COMPOSER_CACHE_VCS_DIR → COMPOSER_CACHE_DIR/vcs
→ XDG/HOME fallbacks), and have GitDriver/HgDriver use it directly.
This brings the Mozart cache layout in line with Composer's: a single
shared vcs root with one subdirectory per sanitized URL.

4 files changed, 29 insertions, 8 deletions

diff --git a/crates/mozart-vcs/src/driver/git.rs b/crates/mozart-vcs/src/driver/git.rs
index 43f4ecb..090a5fa 100644
--- a/crates/mozart-vcs/src/driver/git.rs
+++ b/crates/mozart-vcs/src/driver/git.rs
@@ -27,7 +27,7 @@ impl GitDriver {
     pub fn new(url: &str, config: DriverConfig) -> Self {
         let is_local = Self::is_local_path(url);
         let process = ProcessExecutor::new();
-        let git_util = GitUtil::new(process, config.cache_dir.join("git"));
+        let git_util = GitUtil::new(process, config.cache_vcs_dir.clone());
         Self {
             url: url.to_string(),
             repo_dir: if is_local {
diff --git a/crates/mozart-vcs/src/driver/hg.rs b/crates/mozart-vcs/src/driver/hg.rs
index 0782775..f476e6a 100644
--- a/crates/mozart-vcs/src/driver/hg.rs
+++ b/crates/mozart-vcs/src/driver/hg.rs
@@ -51,8 +51,8 @@ impl HgDriver {
 
 impl VcsDriver for HgDriver {
     async fn initialize(&mut self) -> Result<()> {
-        let cache_dir = self.config.cache_dir.join("hg");
-        std::fs::create_dir_all(&cache_dir)?;
+        let cache_dir = &self.config.cache_vcs_dir;
+        std::fs::create_dir_all(cache_dir)?;
         let repo_dir = cache_dir.join(crate::util::git::GitUtil::sanitize_url(&self.url));
 
         if repo_dir.join(".hg").is_dir() {
diff --git a/crates/mozart-vcs/src/driver/mod.rs b/crates/mozart-vcs/src/driver/mod.rs
index 7a132ed..cfaf11e 100644
--- a/crates/mozart-vcs/src/driver/mod.rs
+++ b/crates/mozart-vcs/src/driver/mod.rs
@@ -34,8 +34,9 @@ pub struct DistReference {
 /// Configuration passed to VCS drivers.
 #[derive(Debug, Clone)]
 pub struct DriverConfig {
-    /// Path for caching VCS mirrors.
-    pub cache_dir: PathBuf,
+    /// Composer's `cache-vcs-dir`: root for VCS mirrors, one
+    /// subdirectory per sanitized repository URL.
+    pub cache_vcs_dir: PathBuf,
     /// GitHub OAuth token (from `GITHUB_TOKEN` or config).
     pub github_token: Option<String>,
     /// GitLab OAuth token.
@@ -53,7 +54,7 @@ pub struct DriverConfig {
 impl Default for DriverConfig {
     fn default() -> Self {
         Self {
-            cache_dir: PathBuf::from(".cache/mozart/vcs"),
+            cache_vcs_dir: default_cache_vcs_dir(),
             github_token: None,
             gitlab_token: None,
             bitbucket_oauth: None,
@@ -64,6 +65,26 @@ impl Default for DriverConfig {
     }
 }
 
+/// Resolve the default `cache-vcs-dir`, honoring Composer's env vars.
+///
+/// Priority: `COMPOSER_CACHE_VCS_DIR` → `COMPOSER_CACHE_DIR/vcs` →
+/// `XDG_CACHE_HOME/mozart/vcs` → `$HOME/.cache/mozart/vcs`.
+fn default_cache_vcs_dir() -> PathBuf {
+    if let Ok(p) = std::env::var("COMPOSER_CACHE_VCS_DIR") {
+        return PathBuf::from(p);
+    }
+    let base = if let Ok(p) = std::env::var("COMPOSER_CACHE_DIR") {
+        PathBuf::from(p)
+    } else if let Ok(xdg) = std::env::var("XDG_CACHE_HOME") {
+        PathBuf::from(xdg).join("mozart")
+    } else if let Ok(home) = std::env::var("HOME") {
+        PathBuf::from(home).join(".cache").join("mozart")
+    } else {
+        PathBuf::from("/tmp").join("mozart")
+    };
+    base.join("vcs")
+}
+
 /// Type of VCS driver.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum DriverType {
diff --git a/crates/mozart-vcs/tests/git_driver_test.rs b/crates/mozart-vcs/tests/git_driver_test.rs
index a8f0ce7..04b224b 100644
--- a/crates/mozart-vcs/tests/git_driver_test.rs
+++ b/crates/mozart-vcs/tests/git_driver_test.rs
@@ -77,7 +77,7 @@ async fn test_git_driver_local_repo() {
     create_test_repo(repo_dir.path());
 
     let config = DriverConfig {
-        cache_dir: cache_dir.path().to_path_buf(),
+        cache_vcs_dir: cache_dir.path().to_path_buf(),
         ..DriverConfig::default()
     };
 
@@ -237,7 +237,7 @@ async fn test_vcs_repository_scan() {
     create_test_repo(repo_dir.path());
 
     let config = DriverConfig {
-        cache_dir: cache_dir.path().to_path_buf(),
+        cache_vcs_dir: cache_dir.path().to_path_buf(),
         ..DriverConfig::default()
     };