fix(security): use OsRng for all security-critical key and token generation (#519)

* fix(security): use OsRng for all security-critical key and token generation

Replace rand::thread_rng() with rand::rngs::OsRng in all security-critical
code paths that generate cryptographic key material, bearer tokens, PKCE
verifiers, CSRF state parameters, and webhook secrets. thread_rng() uses a
userspace CSPRNG (ChaCha) seeded from OS entropy, which is fine for
non-security contexts but adds an unnecessary intermediate layer for
key material where direct OS entropy (OsRng) is the correct choice.

Files changed:
- src/secrets/keychain.rs: master encryption key generation
- src/secrets/crypto.rs: per-secret HKDF salt generation
- src/orchestrator/auth.rs: per-job bearer token generation
- src/channels/web/mod.rs: gateway auth token fallback
- src/cli/oauth_defaults.rs: OAuth PKCE verifier and CSRF state
- src/tools/mcp/auth.rs: MCP OAuth PKCE verifier
- src/extensions/manager.rs: auto-generated extension secrets
- src/setup/channels.rs: webhook secret generation

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

* fix(security): address PR review feedback for OsRng migration

- Remove shadowing inner `use rand::rngs::OsRng` in `generate_salt()`;
  use module-level `aes_gcm::aead::OsRng` import instead (same type,
  avoids divergence risk if rand_core versions drift)
- Fix missed callsites in `pairing/store.rs`: `random_code()` and
  `generate_unique_code()` now use `OsRng` for pairing auth codes
- Add regression tests for `generate_salt()`: correct length,
  non-zero output, uniqueness across calls

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

---------

Co-authored-by: Claude Sonnet 4.6 <[email protected]>

src/secrets/crypto.rs

@@ -59,7 +59,7 @@ impl SecretsCrypto {
     /// Generate a random salt for a new secret.
     pub fn generate_salt() -> Vec<u8> {
         let mut salt = vec![0u8; SALT_SIZE];
-        rand::RngCore::fill_bytes(&mut rand::thread_rng(), &mut salt);
+        rand::RngCore::fill_bytes(&mut OsRng, &mut salt);
         salt
     }
 
@@ -247,4 +247,23 @@ mod tests {
         let decrypted = crypto.decrypt(&encrypted, &salt).unwrap();
         assert_eq!(decrypted.expose().as_bytes(), plaintext.as_slice());
     }
+
+    #[test]
+    fn test_generate_salt_correct_length() {
+        let salt = SecretsCrypto::generate_salt();
+        assert_eq!(salt.len(), super::SALT_SIZE);
+    }
+
+    #[test]
+    fn test_generate_salt_nonzero() {
+        let salt = SecretsCrypto::generate_salt();
+        assert!(salt.iter().any(|&b| b != 0), "salt should not be all zeros");
+    }
+
+    #[test]
+    fn test_generate_salt_unique() {
+        let s1 = SecretsCrypto::generate_salt();
+        let s2 = SecretsCrypto::generate_salt();
+        assert_ne!(s1, s2, "two generated salts should not be identical");
+    }
 }

src/secrets/keychain.rs

@@ -28,8 +28,9 @@ const MASTER_KEY_ACCOUNT: &str = "master_key";
 /// Generate a random 32-byte master key.
 pub fn generate_master_key() -> Vec<u8> {
     use rand::RngCore;
+    use rand::rngs::OsRng;
     let mut key = vec![0u8; 32];
-    rand::thread_rng().fill_bytes(&mut key);
+    OsRng.fill_bytes(&mut key);
     key
 }