feat: add persistent server-client sessions

   Make herdr persistent by default.

   Launching herdr now starts or reattaches to a background session
   server. Clients can detach and reattach while panes and agent
   processes keep running. Session mode now supports multi-client attach,
   auto-detect startup, and a thin-client/headless-server split.

   This also refactors the large app, ui, input, pane, config,
   workspace, and persistence modules into smaller focused submodules,
   while preserving behavior and colocating tests with the code they
   exercise.

   Upgrade notes:
   - persistence mode is now the default
   - in-app quit detaches the current client instead of stopping the server
   - use `herdr server stop` to stop the background session
   - use `--no-session` for the old single-process behavior
   - default socket paths now live under the config directory

1 files changed, 149 insertions, 0 deletions

diff --git a/src/workspace/git.rs b/src/workspace/git.rs
new file mode 100644
index 00000000..ca453003
--- /dev/null
+++ b/src/workspace/git.rs
@@ -0,0 +1,149 @@
+use std::path::{Path, PathBuf};
+
+pub fn derive_label_from_cwd(cwd: &Path) -> String {
+    if let Some(repo_root) = git_repo_root(cwd) {
+        if let Some(name) = repo_root.file_name().and_then(|n| n.to_str()) {
+            return name.to_string();
+        }
+    }
+
+    if let Ok(home) = std::env::var("HOME") {
+        let home = Path::new(&home);
+        if cwd == home {
+            return "~".to_string();
+        }
+    }
+
+    cwd.file_name()
+        .and_then(|n| n.to_str())
+        .filter(|s| !s.is_empty())
+        .map(|s| s.to_string())
+        .unwrap_or_else(|| cwd.display().to_string())
+}
+
+pub fn git_branch(cwd: &Path) -> Option<String> {
+    let repo_root = git_repo_root(cwd)?;
+    let git_dir = git_dir_for_repo_root(&repo_root)?;
+    let head = std::fs::read_to_string(git_dir.join("HEAD")).ok()?;
+    parse_git_head_branch(&head)
+}
+
+fn git_dir_for_repo_root(repo_root: &Path) -> Option<PathBuf> {
+    let git_path = repo_root.join(".git");
+    if git_path.is_dir() {
+        return Some(git_path);
+    }
+
+    let gitdir = std::fs::read_to_string(&git_path).ok()?;
+    let relative = gitdir.trim().strip_prefix("gitdir:")?.trim();
+    let resolved = Path::new(relative);
+    Some(if resolved.is_absolute() {
+        resolved.to_path_buf()
+    } else {
+        repo_root.join(resolved)
+    })
+}
+
+fn parse_git_head_branch(head: &str) -> Option<String> {
+    let branch = head.trim().strip_prefix("ref: refs/heads/")?;
+    (!branch.is_empty()).then(|| branch.to_string())
+}
+
+fn git_repo_root(start: &Path) -> Option<PathBuf> {
+    let mut current = if start.is_dir() {
+        start.to_path_buf()
+    } else {
+        start.parent()?.to_path_buf()
+    };
+
+    loop {
+        if current.join(".git").exists() {
+            return Some(current);
+        }
+        if !current.pop() {
+            return None;
+        }
+    }
+}
+
+pub(super) fn git_ahead_behind(cwd: &Path) -> Option<(usize, usize)> {
+    git_repo_root(cwd)?;
+
+    let output = std::process::Command::new("git")
+        .arg("-C")
+        .arg(cwd)
+        .args(["rev-list", "--left-right", "--count", "HEAD...@{upstream}"])
+        .output()
+        .ok()?;
+
+    if !output.status.success() {
+        return None;
+    }
+
+    let stdout = String::from_utf8(output.stdout).ok()?;
+    parse_git_ahead_behind_output(&stdout)
+}
+
+fn parse_git_ahead_behind_output(stdout: &str) -> Option<(usize, usize)> {
+    let mut parts = stdout.split_whitespace();
+    let ahead = parts.next()?.parse().ok()?;
+    let behind = parts.next()?.parse().ok()?;
+    Some((ahead, behind))
+}
+
+#[cfg(test)]
+mod tests {
+    use std::time::{SystemTime, UNIX_EPOCH};
+
+    use super::*;
+
+    fn temp_test_dir(name: &str) -> PathBuf {
+        let unique = format!(
+            "herdr-workspace-tests-{}-{}-{}",
+            name,
+            std::process::id(),
+            SystemTime::now()
+                .duration_since(UNIX_EPOCH)
+                .unwrap()
+                .as_nanos()
+        );
+        let path = std::env::temp_dir().join(unique);
+        std::fs::create_dir_all(&path).unwrap();
+        path
+    }
+
+    #[test]
+    fn git_branch_reads_head_from_standard_repo() {
+        let root = temp_test_dir("standard-repo");
+        std::fs::create_dir_all(root.join(".git")).unwrap();
+        std::fs::write(root.join(".git/HEAD"), "ref: refs/heads/main\n").unwrap();
+
+        assert_eq!(git_branch(&root).as_deref(), Some("main"));
+
+        std::fs::remove_dir_all(root).unwrap();
+    }
+
+    #[test]
+    fn git_branch_reads_head_from_worktree_gitdir_file() {
+        let root = temp_test_dir("worktree");
+        let worktree_git_dir = root.join(".bare/worktrees/feature");
+        std::fs::create_dir_all(&worktree_git_dir).unwrap();
+        std::fs::write(root.join(".git"), "gitdir: .bare/worktrees/feature\n").unwrap();
+        std::fs::write(worktree_git_dir.join("HEAD"), "ref: refs/heads/feature\n").unwrap();
+
+        assert_eq!(git_branch(&root).as_deref(), Some("feature"));
+
+        std::fs::remove_dir_all(root).unwrap();
+    }
+
+    #[test]
+    fn git_branch_returns_none_for_detached_head() {
+        let root = temp_test_dir("detached-head");
+        std::fs::create_dir_all(root.join(".git")).unwrap();
+        std::fs::write(root.join(".git/HEAD"), "3e1b9a8d\n").unwrap();
+
+        assert_eq!(git_branch(&root), None);
+
+        std::fs::remove_dir_all(root).unwrap();
+    }
+}