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-sh4re/src/priv_proto.rs

@@ -229,29 +229,29 @@ pub enum PrivRequest {
         args: Vec<String>,
     },
 
-    // --- Agent state file writes ---
-    /// Write a credential file into an agent's bind-mounted state directory.
+    // --- Agent credential writes ---
+    /// Write `forge-token` into `AGENT_STATE_ROOT/<agent_name>/state/forge-token`.
     ///
-    /// Path resolved by hive-priv: `AGENT_STATE_ROOT/<agent_name>/state/<filename>`.
-    /// hive-priv validates both the agent name and filename before writing.
-    ///
-    /// After writing, the file is chowned to the agent user's uid/gid
-    /// (read from the state directory's owner) and chmoded 0600 so
-    /// only the agent process can read it.
-    ///
-    /// Required because hive-c0re runs as the unprivileged `hive-core`
-    /// user and cannot write to agent-owned (0755) state directories
-    /// after the privsep that moved c0re from root to a dedicated unix user.
-    WriteAgentStateFile {
+    /// hive-priv validates `agent_name`, creates the state dir if absent,
+    /// writes the file 0600, and chowns it to the state dir's owner so
+    /// the agent process can read it. Required because hive-c0re runs
+    /// unprivileged and cannot write to agent-owned state directories.
+    WriteAgentForgeToken {
         /// Logical agent name (validated by `validate_agent_name`).
         agent_name: String,
-        /// Allowlisted credential filename within the state dir.
-        /// Only `"forge-token"` and `"matrix-token"` are accepted;
-        /// hive-priv rejects any other value.
-        filename: String,
-        /// File content to write. Written as-is; caller is responsible for
-        /// including any trailing newline.
-        content: String,
+        /// Token value. hive-priv appends a trailing newline before writing.
+        token: String,
+    },
+
+    /// Write `matrix-token` into `AGENT_STATE_ROOT/<agent_name>/state/matrix-token`.
+    ///
+    /// Same semantics as `WriteAgentForgeToken` — validates name, creates
+    /// dir, writes 0600, chowns to agent owner.
+    WriteAgentMatrixToken {
+        /// Logical agent name (validated by `validate_agent_name`).
+        agent_name: String,
+        /// Token value. hive-priv appends a trailing newline before writing.
+        token: String,
     },
 }