fix: split WriteAgentStateFile into WriteAgentForgeToken + WriteAgentMatrixToken

Addresses mara's review: each credential type gets its own PrivRequest
variant, making the exact priv surface visible in the wire protocol.
No runtime filename dispatch — the operation name is the gate.

- WriteAgentForgeToken { agent_name, token } → state/forge-token
- WriteAgentMatrixToken { agent_name, token } → state/matrix-token
- priv_client: two typed fns (write_agent_forge_token, write_agent_matrix_token)
- forge.rs: split mint_and_persist_token into mint_and_persist_agent_token
  (priv) + mint_and_persist_core_token (direct write); drop dead token_path fn
- matrix.rs: call write_agent_matrix_token directly

hive-priv/src/main.rs

@@ -322,38 +322,28 @@ async fn exec(req: PrivRequest, writer: &mut OwnedWriteHalf) -> Result<(String,
             run_forge_admin(args).await
         }
 
-        PrivRequest::WriteAgentStateFile {
+        PrivRequest::WriteAgentForgeToken {
             ref agent_name,
-            ref filename,
-            ref content,
+            ref token,
         } => {
             validate_agent_name(agent_name)?;
-            validate_state_filename(filename)?;
-            write_agent_state_file(agent_name, filename, content)
+            write_agent_state_file(agent_name, "forge-token", &format!("{token}\n"))
+        }
+
+        PrivRequest::WriteAgentMatrixToken {
+            ref agent_name,
+            ref token,
+        } => {
+            validate_agent_name(agent_name)?;
+            write_agent_state_file(agent_name, "matrix-token", &format!("{token}\n"))
         }
     }
 }
 
-/// Explicit allowlist of filenames `WriteAgentStateFile` may write.
-/// New credential files must be added here deliberately — hive-priv
-/// rejects any filename not in this list.
-const ALLOWED_STATE_FILENAMES: &[&str] = &["forge-token", "matrix-token"];
-
-/// Validate a filename destined for `WriteAgentStateFile` against the
-/// explicit allowlist. Only known credential filenames are accepted.
-fn validate_state_filename(filename: &str) -> Result<()> {
-    if !ALLOWED_STATE_FILENAMES.contains(&filename) {
-        bail!(
-            "state filename {filename:?} not in allowlist {:?}",
-            ALLOWED_STATE_FILENAMES
-        );
-    }
-    Ok(())
-}
-
-/// Write `content` to `AGENT_STATE_ROOT/<agent_name>/state/<filename>`,
-/// chown to the agent user (derived from the state dir's existing owner),
-/// and chmod 0600. Running as root (hive-priv), so all of this succeeds
+/// Shared helper for `WriteAgentForgeToken` and `WriteAgentMatrixToken`.
+/// Writes `content` to `AGENT_STATE_ROOT/<agent_name>/state/<filename>`,
+/// chowns to the agent user (derived from the state dir's existing owner),
+/// and chmods 0600. Running as root (hive-priv), so this succeeds
 /// regardless of the file's prior owner/permissions.
 fn write_agent_state_file(
     agent_name: &str,