fix(deps): override marked-terminal peer to accept marked 18

marked-terminal@7.3.0 declares peer marked '>=1 <16' and has no release
supporting marked 16+, so the bare bump made `npm install` fail ERESOLVE
(CI runs `just setup` -> `npm install`). Add an npm `overrides` entry
pinning marked-terminal's marked to the root `$marked` (18). The terminal
renderer is the sole `marked` consumer (src/agent/markdown-renderer.ts)
and its 12 regression tests + the full suite pass under marked 18.

Signed-off-by: Simon Davies <simongdavies@users.noreply.github.com>

Author: simongdavies

package.json

@@ -55,5 +55,10 @@
     "tsx": "^4.0.0",
     "typescript": "^5.8.0",
     "vitest": "^4.0.18"
+  },
+  "overrides": {
+    "marked-terminal": {
+      "marked": "$marked"
+    }
   }
 }