ci(mantis): avoid host apt in telegram proof

.github/codex/prompts/mantis-telegram-desktop-proof.md

@@ -45,8 +45,8 @@ Required workflow:
    install and build each worktree with the repo's normal `pnpm` commands.
 5. In each worktree, run the real-user Telegram Crabbox proof flow from the
    skill. Use `scripts/e2e/telegram-user-driver.py`, the workflow-provided
-   `crabbox` binary, and the workflow-provided local media tools; do not
-   generate or patch replacement proof tooling during the run. Use the same
+   `crabbox` binary, and the workflow-checked local `ffmpeg`; do not generate,
+   install, or patch replacement proof tooling during the run. Use the same
    proof idea for baseline and candidate. You may iterate and rerun if the
    visual result is not convincing.
 6. Open Telegram Desktop directly to the newest relevant message with the

.github/workflows/mantis-telegram-desktop-proof.yml

@@ -331,15 +331,13 @@ jobs:
           crabbox --version
           crabbox media preview --help >/dev/null
 
-      - name: Install local proof tools
+      - name: Check local proof tools
         shell: bash
         run: |
           set -euo pipefail
-          sudo apt-get update -y
-          sudo DEBIAN_FRONTEND=noninteractive apt-get install -y ffmpeg gifsicle
           test -f scripts/e2e/telegram-user-driver.py
+          command -v ffmpeg
           ffmpeg -version >/dev/null
-          gifsicle --version >/dev/null
 
       - name: Ensure agent key exists
         env: