Honor GSTACK_CHROMIUM_PATH on the headless launch and setup paths

[ryanlaiwy]

Problem

GSTACK_CHROMIUM_PATH lets you point gstack at a custom Chromium binary. The headed path (launchPersistentContext) already honors it, but two paths ignored it:

• BrowserManager's headless chromium.launch() — a custom binary was used in headed mode only.
• setup's ensure_playwright_browser self-test — always launched Playwright's bundled Chromium.

On NixOS the bundled chrome-headless-shell can't run (it expects /lib64/ld-linux-x86-64.so.2, absent on a Nix filesystem), so ./setup bails before registering sub-skills and headless browse fails — even with GSTACK_CHROMIUM_PATH set to a working system Chromium.

Change

• Pass executablePath on the headless chromium.launch() when GSTACK_CHROMIUM_PATH is set.
• ensure_playwright_browser verifies that same binary (with --no-sandbox) when the env var is set.

Both are no-ops when GSTACK_CHROMIUM_PATH is unset — zero impact on normal installs.

Testing

Verified on a NixOS-style container (Replit): with GSTACK_CHROMIUM_PATH pointed at the system Chromium, ./setup completes and registers all sub-skills, and headless browse navigates and screenshots correctly. Without the env var, behavior is unchanged.

🤖 Generated with Claude Code

[jbetala7]

The setup self-test half looks useful, but the browser-manager.ts half needs to pick up the same baked-extension guard that #1614 added after review.

Right now this adds executablePath: process.env.GSTACK_CHROMIUM_PATH to the headless/off-screen launch path while leaving the if (extensionsDir) block unconditional. With BROWSE_EXTENSIONS_DIR plus a GBrowser/GStack Browser binary that already has the extension baked in, that path still passes --disable-extensions-except / --load-extension and can double-load the extension. launchHeaded() already avoids that via isCustomChromium(), and #1614 now mirrors that guard for the headless path.

Can this either rebase/port #1614's isCustomChromium() guard and add a regression for GSTACK_CHROMIUM_KIND=custom-extension-baked or a GBrowser path, or split this down to just the setup verification change if #1614 remains the canonical browser-manager PR?