fix(agent): SPEC-018 review fixes — agent_id persistence, managed fallback, HKEY typing

Address the SPEC-018 Phase 1 code review (reports/2026-06-03-spec018-review.md):

- Bug 2 (config.rs): stop agent_id churn on every restart. The embedded-config
  path always wins in Config::load, so the saved agent_id was never read back.
  Add Config::persisted_agent_id() and reuse a prior id from the TOML; only mint
  a new UUID when none exists.
- Bug 1 (main.rs): remove the non-functional in-process fallback in
  run_permanent_agent_managed. A managed agent's cak_ store is SYSTEM-only ACL'd,
  so a non-elevated in-process run cannot authenticate (load_cak permission-denied,
  or enroll C1 read-back failure). Return an actionable "install elevated" error
  instead of pretending to provide an agent; update the misleading comments.
- Issue 6 (startup.rs): replace the fragile transmute::<HANDLE, HKEY> with the
  windows crate's typed HKEY out-param; add SAFETY comments.

cargo check -p guruconnect --target x86_64-pc-windows-msvc passes clean.
Deferred lower-severity items tracked in #8.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent/src/startup.rs

@@ -9,7 +9,7 @@ use tracing::{info, warn};
 use windows::core::PCWSTR;
 #[cfg(windows)]
 use windows::Win32::System::Registry::{
-    RegCloseKey, RegDeleteValueW, RegOpenKeyExW, RegSetValueExW, HKEY_CURRENT_USER, KEY_WRITE,
+    RegCloseKey, RegDeleteValueW, RegOpenKeyExW, RegSetValueExW, HKEY, HKEY_CURRENT_USER, KEY_WRITE,
     REG_SZ,
 };
 
@@ -42,40 +42,39 @@ pub fn add_to_startup() -> Result<()> {
         .chain(std::iter::once(0))
         .collect();
 
+    // SAFETY: FFI into the Win32 registry API. `key_path`/`value_name`/`value_data`
+    // are NUL-terminated wide strings that outlive the calls. `RegOpenKeyExW`
+    // writes the opened key into `hkey`; we only use it after confirming success,
+    // and always pair it with `RegCloseKey`.
     unsafe {
-        let mut hkey = windows::Win32::Foundation::HANDLE::default();
+        let mut hkey = HKEY::default();
 
-        // Open the Run key
+        // Open the Run key. RegOpenKeyExW takes a `*mut HKEY` out-param.
         let result = RegOpenKeyExW(
             HKEY_CURRENT_USER,
             PCWSTR(key_path.as_ptr()),
             0,
             KEY_WRITE,
-            &mut hkey as *mut _ as *mut _,
+            &mut hkey,
         );
 
         if result.is_err() {
             anyhow::bail!("Failed to open registry key: {:?}", result);
         }
 
-        let hkey_raw = std::mem::transmute::<
-            windows::Win32::Foundation::HANDLE,
-            windows::Win32::System::Registry::HKEY,
-        >(hkey);
-
         // Set the value
         let data_bytes =
             std::slice::from_raw_parts(value_data.as_ptr() as *const u8, value_data.len() * 2);
 
         let set_result = RegSetValueExW(
-            hkey_raw,
+            hkey,
             PCWSTR(value_name.as_ptr()),
             0,
             REG_SZ,
             Some(data_bytes),
         );
 
-        let _ = RegCloseKey(hkey_raw);
+        let _ = RegCloseKey(hkey);
 
         if set_result.is_err() {
             anyhow::bail!("Failed to set registry value: {:?}", set_result);
@@ -103,15 +102,19 @@ pub fn remove_from_startup() -> Result<()> {
         .chain(std::iter::once(0))
         .collect();
 
+    // SAFETY: FFI into the Win32 registry API. `key_path`/`value_name` are
+    // NUL-terminated wide strings that outlive the calls. `RegOpenKeyExW` writes
+    // the opened key into `hkey`; we only use it after confirming success, and
+    // always pair it with `RegCloseKey`.
     unsafe {
-        let mut hkey = windows::Win32::Foundation::HANDLE::default();
+        let mut hkey = HKEY::default();
 
         let result = RegOpenKeyExW(
             HKEY_CURRENT_USER,
             PCWSTR(key_path.as_ptr()),
             0,
             KEY_WRITE,
-            &mut hkey as *mut _ as *mut _,
+            &mut hkey,
         );
 
         if result.is_err() {
@@ -119,14 +122,9 @@ pub fn remove_from_startup() -> Result<()> {
             return Ok(()); // Not an error if key doesn't exist
         }
 
-        let hkey_raw = std::mem::transmute::<
-            windows::Win32::Foundation::HANDLE,
-            windows::Win32::System::Registry::HKEY,
-        >(hkey);
+        let delete_result = RegDeleteValueW(hkey, PCWSTR(value_name.as_ptr()));
 
-        let delete_result = RegDeleteValueW(hkey_raw, PCWSTR(value_name.as_ptr()));
-
-        let _ = RegCloseKey(hkey_raw);
+        let _ = RegCloseKey(hkey);
 
         if delete_result.is_err() {
             warn!("Registry value may not exist: {:?}", delete_result);