fix: resolve PTY failure, MMDS file writes, and metrics instability in envd-rs

Three bugs fixed:

1. PTY connections failed because home directory was hardcoded as
   /home/{username} instead of reading from /etc/passwd. For root,
   this produced /home/root/ which doesn't exist — CWD validation
   rejected every PTY Start request without explicit cwd. Fixed all
   6 locations to use user.dir from nix::unistd::User.

2. MMDS polling silently failed to parse metadata because the
   logs_collector_address field lacked #[serde(default)]. The host
   agent only sends instanceID + envID — missing "address" field
   caused every deserialize attempt to fail, so .WRENN_SANDBOX_ID
   and .WRENN_TEMPLATE_ID were never written. Also added error
   logging and create_dir_all before file writes.

3. Metrics CPU values were non-deterministic because a fresh
   sysinfo::System was created per request with a 100ms sleep
   between reads. Replaced with a background thread that samples
   CPU at fixed 1-second intervals via a persistent System instance,
   matching gopsutil's internal caching behavior. Metrics endpoint
   now reads cached atomic values — no blocking, consistent window.

Also: close master PTY fd in child pre_exec, add process.Start
request logging, bump version to 0.2.0.

envd-rs/src/rpc/process_handler.rs

@@ -141,7 +141,7 @@ pub fn spawn_process(
 ) -> Result<Arc<ProcessHandle>, ConnectError> {
     let mut env: Vec<(String, String)> = Vec::new();
     env.push(("PATH".into(), std::env::var("PATH").unwrap_or_default()));
-    let home = format!("/home/{}", user.name);
+    let home = user.dir.to_string_lossy().to_string();
     env.push(("HOME".into(), home));
     env.push(("USER".into(), user.name.clone()));
     env.push(("LOGNAME".into(), user.name.clone()));
@@ -206,7 +206,9 @@ pub fn spawn_process(
         unsafe {
             use std::os::unix::io::AsRawFd;
             let slave_raw = slave_fd.as_raw_fd();
+            let master_raw = master_fd.as_raw_fd();
             command.pre_exec(move || {
+                libc::close(master_raw);
                 nix::unistd::setsid()
                     .map_err(|e| std::io::Error::new(std::io::ErrorKind::Other, e))?;
                 libc::ioctl(slave_raw, libc::TIOCSCTTY, 0);