improve: enhance error message for non-git repositories

Add user-friendly error message when git chain is run outside a git repository:
- Replace technical git2 error with clear, actionable message
- Add helpful hints directing users to run 'git init'
- Style error output consistently with colored formatting (error: in red, hint: in yellow)
- Mirror Git's own error message style for familiarity

Add integration test for non-git repository edge case:
- Test in system temp directory to avoid finding parent repos
- Verify proper error message and exit status
- Include diagnostic printing following CLAUDE.md guidelines

All 53 tests passing, make ci-local verified.

Author: dashed

src/git_chain/core.rs

@@ -1,7 +1,7 @@
 use std::process;
 
 use colored::*;
-use git2::{BranchType, Config, ConfigLevel, Error, ErrorCode, ObjectType, Repository};
+use git2::{BranchType, Config, ConfigLevel, Error, ErrorClass, ErrorCode, ObjectType, Repository};
 use regex::Regex;
 
 use super::GitChain;
@@ -12,7 +12,28 @@ impl GitChain {
     pub fn init() -> Result<Self, Error> {
         let name_of_current_executable = executable_name();
 
-        let repo = Repository::discover(".")?;
+        let repo = match Repository::discover(".") {
+            Ok(repo) => repo,
+            Err(ref e)
+                if e.class() == ErrorClass::Repository && e.code() == ErrorCode::NotFound =>
+            {
+                eprintln!(
+                    "{} Not a git repository (or any of the parent directories)",
+                    "error:".red().bold()
+                );
+                eprintln!(
+                    "\n{} This command must be run inside a git repository.",
+                    "hint:".yellow().bold()
+                );
+                eprintln!(
+                    "{} Run {} to create a new git repository.",
+                    "hint:".yellow().bold(),
+                    "git init".bold()
+                );
+                process::exit(1);
+            }
+            Err(e) => return Err(e),
+        };
 
         if repo.is_bare() {
             eprintln!(

tests/misc.rs

@@ -1,6 +1,8 @@
 #[path = "common/mod.rs"]
 pub mod common;
 
+use std::fs;
+
 use common::{
     create_new_file, first_commit_all, generate_path_to_repo, get_current_branch_name,
     run_test_bin_expect_err, setup_git_repo, teardown_git_repo,
@@ -31,3 +33,55 @@ fn no_subcommand() {
 
     teardown_git_repo(repo_name);
 }
+
+#[test]
+fn not_a_git_repo() {
+    // Create a directory in the system temp location to avoid finding parent git repos
+    let temp_dir = std::env::temp_dir();
+    let path_to_non_git_dir = temp_dir.join("git_chain_test_not_a_repo");
+
+    // Create a directory that is NOT a git repository
+    fs::remove_dir_all(&path_to_non_git_dir).ok();
+    fs::create_dir_all(&path_to_non_git_dir).unwrap();
+
+    // Run git chain in the non-git directory
+    let args: Vec<String> = vec![];
+    let output = run_test_bin_expect_err(&path_to_non_git_dir, args);
+
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    let stderr = String::from_utf8_lossy(&output.stderr);
+
+    // Diagnostic printing
+    println!("=== TEST DIAGNOSTICS ===");
+    println!("Test directory: {:?}", path_to_non_git_dir);
+    println!("STDOUT: {}", stdout);
+    println!("STDERR: {}", stderr);
+    println!("EXIT STATUS: {}", output.status);
+    println!("Is directory a git repo: false (intentional test condition)");
+    println!("======");
+
+    // Uncomment to stop test execution and inspect state with captured output
+    // assert!(false, "DEBUG STOP: not_a_git_repo test section");
+    // assert!(false, "stdout: {}", stdout);
+    // assert!(false, "stderr: {}", stderr);
+    // assert!(false, "status code: {}", output.status.code().unwrap_or(0));
+
+    // Specific assertions based on expected behavior
+    assert!(
+        !output.status.success(),
+        "Command should fail when run in non-git directory"
+    );
+    assert!(
+        stderr.contains("Not a git repository"),
+        "Error message should mention 'Not a git repository', got: {}",
+        stderr
+    );
+    assert!(
+        stderr.contains("This command must be run inside a git repository"),
+        "Error message should provide helpful hint, got: {}",
+        stderr
+    );
+
+    // Clean up
+    fs::remove_dir_all(&path_to_non_git_dir).ok();
+}