Fix agent rules files for remote project by loading via buffer (#29440)

When using the agent with a project shared by a collaborator, rules file
loading didn't work as it was trying to read from the client's
filesystem

Release Notes:

- Fixed rules file loading when using the agent with a project shared by
a collaborator.

crates/worktree/src/worktree.rs

@@ -5549,6 +5549,10 @@ impl ProjectEntryId {
         self.0 as u64
     }
 
+    pub fn from_usize(id: usize) -> Self {
+        ProjectEntryId(id)
+    }
+
     pub fn to_usize(&self) -> usize {
         self.0
     }