fix(codex-config): stop rejecting env values that start with "-"

--env's value is the RHS of one KEY=VALUE argv element, never its own
argv position, so a leading "-" can never be parsed as a flag — unlike
the key, which stays validated as an identifier. assertArg's leading-dash
guard was reused for the value too, refusing ordinary values like "-1".
Split it into assertEnvValue (no leading-dash check) for values only.

server/__tests__/codex-cli.test.js

@@ -189,6 +189,20 @@ describe("codex-cli argv construction", () => {
     }
   });
 
+  it("accepts an env value starting with '-' — it's the RHS of one KEY=VALUE argv element, never a flag", async () => {
+    const restore = stubCodex();
+    try {
+      const result = await cli.mcpAdd({
+        name: "docs",
+        command: "node",
+        env: { OFFSET: "-1" },
+      });
+      assert.deepEqual(argvOf(result), ["mcp", "add", "docs", "--env", "OFFSET=-1", "--", "node"]);
+    } finally {
+      restore();
+    }
+  });
+
   it("builds mcp list / remove argvs", async () => {
     const restore = stubCodex();
     try {