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/state.rs

@@ -1,4 +1,4 @@
-use std::sync::atomic::AtomicBool;
+use std::sync::atomic::{AtomicBool, AtomicU32, Ordering};
 use std::sync::Arc;
 
 use crate::auth::token::SecureToken;
@@ -17,6 +17,8 @@ pub struct AppState {
     pub access_token: SecureToken,
     pub conn_tracker: ConnTracker,
     pub port_subsystem: Option<Arc<PortSubsystem>>,
+    pub cpu_used_pct: AtomicU32,
+    pub cpu_count: AtomicU32,
 }
 
 impl AppState {
@@ -27,7 +29,7 @@ impl AppState {
         is_fc: bool,
         port_subsystem: Option<Arc<PortSubsystem>>,
     ) -> Arc<Self> {
-        Arc::new(Self {
+        let state = Arc::new(Self {
             defaults,
             version,
             commit,
@@ -37,6 +39,49 @@ impl AppState {
             access_token: SecureToken::new(),
             conn_tracker: ConnTracker::new(),
             port_subsystem,
-        })
+            cpu_used_pct: AtomicU32::new(0),
+            cpu_count: AtomicU32::new(0),
+        });
+
+        let state_clone = Arc::clone(&state);
+        std::thread::spawn(move || {
+            cpu_sampler(state_clone);
+        });
+
+        state
+    }
+
+    pub fn cpu_used_pct(&self) -> f32 {
+        f32::from_bits(self.cpu_used_pct.load(Ordering::Relaxed))
+    }
+
+    pub fn cpu_count(&self) -> u32 {
+        self.cpu_count.load(Ordering::Relaxed)
+    }
+}
+
+fn cpu_sampler(state: Arc<AppState>) {
+    use sysinfo::System;
+
+    let mut sys = System::new();
+    sys.refresh_cpu_all();
+
+    loop {
+        std::thread::sleep(std::time::Duration::from_secs(1));
+        sys.refresh_cpu_all();
+
+        let pct = sys.global_cpu_usage();
+        let rounded = if pct > 0.0 {
+            (pct * 100.0).round() / 100.0
+        } else {
+            0.0
+        };
+
+        state
+            .cpu_used_pct
+            .store(rounded.to_bits(), Ordering::Relaxed);
+        state
+            .cpu_count
+            .store(sys.cpus().len() as u32, Ordering::Relaxed);
     }
 }