fix(backend): install agent-browser + Chromium in Docker image (#12301)

The Copilot browser tool (`browser_navigate`, `browser_act`,
`browser_screenshot`) has been broken on dev because `agent-browser` CLI
+ Chromium were never installed in the backend Docker image.

### Changes 🏗️

- Added `npx playwright install-deps chromium` to install Chromium
runtime libraries (libnss3, libatk, etc.)
- Added `npm install -g agent-browser` to install the CLI
- Added `agent-browser install` to download the Chromium binary
- Layer is placed after existing COPY-from-builder lines to preserve
Docker cache ordering

### Root cause

Every `browser_navigate` call fails with:
```
WARNING  [browser_navigate] open failed for <url>: agent-browser is not installed
(run: npm install -g agent-browser && agent-browser install).
```
The error originates from `FileNotFoundError` in `agent_browser.py:101`
when the subprocess tries to execute the `agent-browser` binary which
doesn't exist in the container.

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [x] Verified `agent-browser` binary is missing from current dev pod
via `kubectl logs`
- [x] Confirmed session `01eeac29-5a7` shows repeated failures for all
URLs
- [ ] After deploy: verify browser_navigate works in a Copilot session
on dev

#### For configuration changes:
- [x] `.env.default` is updated or already compatible with my changes
- [x] `docker-compose.yml` is updated or already compatible with my
changes
- [x] I have included a list of my configuration changes in the PR
description (under **Changes**)

autogpt_platform/backend/Dockerfile

@@ -111,13 +111,29 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 # Copy poetry (build-time only, for `poetry install --only-root` to create entry points)
 COPY --from=builder /usr/local/lib/python3* /usr/local/lib/python3*
 COPY --from=builder /usr/local/bin/poetry /usr/local/bin/poetry
-# Copy Node.js installation for Prisma
+# Copy Node.js installation for Prisma and agent-browser.
+# npm/npx are symlinks in the builder (-> ../lib/node_modules/npm/bin/*-cli.js);
+# COPY resolves them to regular files, breaking require() paths.  Recreate as
+# proper symlinks so npm/npx can find their modules.
 COPY --from=builder /usr/bin/node /usr/bin/node
 COPY --from=builder /usr/lib/node_modules /usr/lib/node_modules
-COPY --from=builder /usr/bin/npm /usr/bin/npm
-COPY --from=builder /usr/bin/npx /usr/bin/npx
+RUN ln -s ../lib/node_modules/npm/bin/npm-cli.js /usr/bin/npm \
+    && ln -s ../lib/node_modules/npm/bin/npx-cli.js /usr/bin/npx
 COPY --from=builder /root/.cache/prisma-python/binaries /root/.cache/prisma-python/binaries
 
+# Install agent-browser (Copilot browser tool) + Chromium runtime dependencies.
+# These are the runtime libraries Chromium/Playwright needs on Debian 13 (trixie).
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libnss3 libnspr4 libatk1.0-0 libatk-bridge2.0-0 libcups2 libdrm2 \
+    libdbus-1-3 libxkbcommon0 libatspi2.0-0t64 libxcomposite1 libxdamage1 \
+    libxfixes3 libxrandr2 libgbm1 libasound2t64 libpango-1.0-0 libcairo2 \
+    libx11-6 libx11-xcb1 libxcb1 libxext6 libglib2.0-0t64 \
+    fonts-liberation libfontconfig1 \
+    && rm -rf /var/lib/apt/lists/* \
+    && npm install -g agent-browser \
+    && agent-browser install \
+    && rm -rf /tmp/* /root/.npm
+
 WORKDIR /app/autogpt_platform/backend
 
 # Copy only the .venv from builder (not the entire /app directory)