Add worktree settings parser (#40624)

## What changed

- Add the `codex-worktree` crate for resolving managed worktree settings from
  the existing `[desktop]` configuration.
- Default the worktree root to `$CODEX_HOME/worktrees`, automatic cleanup to
  enabled, and retained worktrees to 15.
- Validate configured roots, cleanup flags, and retention counts before
  exposing the effective settings.

## Testing

- Cover defaults, configured values, and invalid root and retention settings.

GitOrigin-RevId: 1b279c62ef41e3c8eb540ce9beaf0f6cd96d903f

codex-rs/worktree/src/lib.rs

@@ -0,0 +1,4 @@
+mod settings;
+
+pub use settings::DEFAULT_WORKTREE_KEEP_COUNT;
+pub use settings::WorktreeSettings;

codex-rs/worktree/src/settings.rs

@@ -0,0 +1,83 @@
+//! Parses and validates worktree settings from the existing `[desktop]` config.
+//!
+//! Resolves the managed worktree root and the automatic-cleanup and retention
+//! settings without introducing a separate configuration format.
+
+use anyhow::Context;
+use anyhow::Result;
+use anyhow::bail;
+use serde_json::Value;
+use std::collections::HashMap;
+use std::path::Path;
+use std::path::PathBuf;
+
+const WORKTREE_ROOT: &str = "git-worktree-root";
+const AUTO_CLEANUP: &str = "worktree-auto-cleanup-enabled";
+const KEEP_COUNT: &str = "worktree-keep-count";
+
+pub const DEFAULT_WORKTREE_KEEP_COUNT: usize = 15;
+
+/// Effective host-local settings already understood by Codex Desktop.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct WorktreeSettings {
+    pub root: PathBuf,
+    pub auto_cleanup_enabled: bool,
+    pub keep_count: usize,
+}
+
+impl WorktreeSettings {
+    /// Resolves existing `[desktop]` values without introducing another config format.
+    pub fn from_desktop_config(
+        codex_home: &Path,
+        desktop: Option<&HashMap<String, Value>>,
+    ) -> Result<Self> {
+        let root = match desktop.and_then(|settings| settings.get(WORKTREE_ROOT)) {
+            None | Some(Value::Null) => codex_home.join("worktrees"),
+            Some(value) => {
+                let configured = value
+                    .as_str()
+                    .context("desktop.git-worktree-root must be a string")?
+                    .trim();
+                if configured.is_empty() {
+                    codex_home.join("worktrees")
+                } else {
+                    let path = PathBuf::from(configured);
+                    if !path.is_absolute() {
+                        bail!("desktop.git-worktree-root must be an absolute path");
+                    }
+                    path
+                }
+            }
+        };
+
+        let auto_cleanup_enabled = desktop
+            .and_then(|settings| settings.get(AUTO_CLEANUP))
+            .map(|value| {
+                value
+                    .as_bool()
+                    .context("desktop.worktree-auto-cleanup-enabled must be a boolean")
+            })
+            .transpose()?
+            .unwrap_or(true);
+
+        let keep_count = desktop
+            .and_then(|settings| settings.get(KEEP_COUNT))
+            .map(|value| {
+                let count = value
+                    .as_u64()
+                    .context("desktop.worktree-keep-count must be a positive integer")?;
+                if count == 0 {
+                    bail!("desktop.worktree-keep-count must be a positive integer");
+                }
+                usize::try_from(count).context("desktop.worktree-keep-count is too large")
+            })
+            .transpose()?
+            .unwrap_or(DEFAULT_WORKTREE_KEEP_COUNT);
+
+        Ok(Self {
+            root: dunce::simplified(&root).to_path_buf(),
+            auto_cleanup_enabled,
+            keep_count,
+        })
+    }
+}