fix(cli): aura new skips git init inside an existing work tree (0107 fieldtest, orthogonal)

The cargo-new mirror completed: a scaffold landing inside a tracked
tree no longer nests a .git (the gitlink/submodule hazard that bit the
fieldtest corpus); outside a work tree the best-effort init is
unchanged. Oracle: git rev-parse --is-inside-work-tree in the target
dir; any failure reads as not-in-a-work-tree so the init stays
best-effort.

RED-first (tdd-author handoff): the inside-a-work-tree pin observed
failing on the nested .git; the outside guard pinned against
over-removal.

Gates: workspace 1003/0, clippy -D warnings clean.

closes #204

crates/aura-cli/src/scaffold.rs

@@ -239,6 +239,12 @@ pub fn scaffold(spec: &ScaffoldSpec) -> Result<(), String> {
     write("blueprints/signal.json", render(SIGNAL_JSON, spec))?;
     write("CLAUDE.md", render(CLAUDE_MD, spec))?;
     // Best-effort git init: a warning, never an error (cargo-new mirror).
+    // Also the cargo-new mirror: inside an existing work tree the init is
+    // SKIPPED silently — a nested .git is a gitlink/submodule hazard when the
+    // scaffold lands in a tracked tree (#204).
+    if inside_git_work_tree(&spec.target_dir) {
+        return Ok(());
+    }
     match std::process::Command::new("git")
         .arg("init")
         .current_dir(&spec.target_dir)
@@ -250,6 +256,18 @@ pub fn scaffold(spec: &ScaffoldSpec) -> Result<(), String> {
     Ok(())
 }
 
+/// Whether `dir` already sits inside a git work tree (`git rev-parse`, the
+/// same oracle cargo-new consults). A missing git binary or any failure reads
+/// as "not in a work tree" — the init stays best-effort either way.
+fn inside_git_work_tree(dir: &std::path::Path) -> bool {
+    std::process::Command::new("git")
+        .args(["rev-parse", "--is-inside-work-tree"])
+        .current_dir(dir)
+        .output()
+        .map(|o| o.status.success() && String::from_utf8_lossy(&o.stdout).trim() == "true")
+        .unwrap_or(false)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;