Fix Playwright and Backlog MCPs not using their configured defaults

[szachovy]

Summary

Fixes #33 — two devcontainer MCPs weren't behaving as configured:

• Playwright: --browser chromium internally resolves to Playwright's chrome-for-testing channel (a separately-managed/downloaded build), not the apt-installed /usr/bin/chromium. DISPLAY=":99" was set but no Xvfb server was ever started anywhere in the container lifecycle (the xvfb package was installed in Pin TruffleHog version, fix firewall DNS, remove instructions skill #29 as a workaround but never wired up). Result: headed launches crashed with "Missing X server", and Playwright fell back to downloading its own Chrome build — matching the "downloads chrome (or chrome-testing), initially fails, eventually succeeds" behavior in the issue. Traced this to a regression: Install curated Claude plugins; add headless Chromium (#16) #27 originally pinned the system Chromium via a Dockerfile env var, Pin TruffleHog version, fix firewall DNS, remove instructions skill #29 silently dropped it.
• Backlog MCP: @radleta/backlog-md-mcp resolves the project directory via process.env.PWD || process.cwd(), preferring the env var. In this container PWD is inherited stale from the VS Code server process chain (not /workspace), even though the actual OS cwd is correct — so it always reports "Backlog.md is not initialized" and callers fall back to the raw CLI.

Both root causes were empirically verified (live reproduction, source tracing into the installed @playwright/mcp / @radleta/backlog-md-mcp packages, and git log -S for the Dockerfile regression) before fixing.

Changes

• .devcontainer/Dockerfile: remove the dead xvfb package.
• .devcontainer/config/{claude,codex,opencode}/*: playwright entries now pass --executable-path /usr/bin/chromium --headless and drop DISPLAY; backlog-md entries now set PWD=/workspace.
• CHANGELOG.md: documented both fixes under [Unreleased].

Test plan

• Validated JSON syntax of the two edited JSON configs (jq .)
• Confirmed the pre-existing TOML file's multi-line inline table (unrelated to this change) is the only reason Python's strict tomllib rejects it; diff itself is well-formed
• Live-reproduced the original crash (Missing X server or $DISPLAY, downloading from ~/.cache/ms-playwright/chromium-*), then live-verified the fix: started @playwright/mcp with the new args/env and drove a real browser_navigate call over the MCP protocol — succeeded instantly, no download, no display needed
• Verified isBacklogInitialized('/workspace') returns true while the stale inherited PWD returns false, confirming PWD=/workspace resolves the detection bug

[szachovy]

Check if I can use Chrome for testing: https://www.chromium.org/getting-involved/download-chromium/ with playwright