fix: restrict WriteAgentStateFile to explicit filename allowlist

Addresses mara's security review: replace validate_state_filename (which
accepted any non-traversal filename) with a tight allowlist containing
only the two known credential filenames: forge-token and matrix-token.

Also addresses argus review feedback:
- drop issue tag from priv_proto.rs doc comment
- add comment explaining the path-detection heuristic in forge.rs
- add note about create_dir_all uid=0 edge case in write_agent_state_file

hive-sh4re/src/priv_proto.rs

@@ -241,12 +241,13 @@ pub enum PrivRequest {
     ///
     /// Required because hive-c0re runs as the unprivileged `hive-core`
     /// user and cannot write to agent-owned (0755) state directories
-    /// after the privsep introduced in #702.
+    /// after the privsep that moved c0re from root to a dedicated unix user.
     WriteAgentStateFile {
         /// Logical agent name (validated by `validate_agent_name`).
         agent_name: String,
-        /// Plain filename within the state dir — no path separators allowed.
-        /// Example: `"forge-token"`, `"matrix-token"`.
+        /// 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.