fix(scripts): extract cross-platform openBrowser helper with structured launch result (#3180)

scripts/plan-canvas.js and scripts/control-pane.js both define their own
openBrowser helpers for launching the user's default browser. The two
implementations have diverged:

- plan-canvas.js dispatches across darwin/win32/linux but its
  try/spawn try/catch does not catch async child errors (ENOENT/EACCES
  on hosts without the launcher command). Returns a bare true/false.
- control-pane.js is darwin-only and silently returns early on
  Windows/Linux, so the two scripts behave inconsistently across
  platforms.

When the launcher command is missing (e.g. headless CI without xdg-open)
the JSON output still says 'browser: opened', lying to the agent.

Changes:
- scripts/lib/platform-launch.js: shared openBrowser helper that
  dispatches per platform, wires child 'error' so ENOENT/EACCES
  propagate, and returns {opened, reason} instead of a bare boolean.
- scripts/plan-canvas.js: drops its local helper, imports the shared
  one, and adds browserReason to its JSON output.
- scripts/control-pane.js: replaces its darwin-only branch with the
  shared helper and logs the structured reason on failure.
- tests/lib/platform-launch.test.js: 7 node:test cases covering
  opener-command selection, invalid-URL guard, structured-result
  shape, and a smoke run for the actual spawn.

Verification: node --test tests/lib/platform-launch.test.js → 7/7 pass.

Co-authored-by: auyua9 <auyua9@users.noreply.github.com>

scripts/control-pane.js

@@ -10,16 +10,14 @@ const {
 } = require('./lib/control-pane/server');
 const { describeMissingDependencyError } = require('./lib/missing-dependency');
 
+// openBrowser is now in scripts/lib/platform-launch.js — keep a thin wrapper
+// for backwards compatibility, but surface the structured result.
+const { openBrowser: launchOpenBrowser } = require('./lib/platform-launch');
 function openBrowser(url) {
-  if (process.platform !== 'darwin') return;
-  const child = spawn('open', [url], {
-    stdio: 'ignore',
-    detached: true,
-  });
-  child.on('error', error => {
-    console.error(`[control-pane] failed to open browser: ${error.message}`);
-  });
-  child.unref();
+  const result = launchOpenBrowser(url);
+  if (!result.opened) {
+    console.error(`[control-pane] failed to open browser: ${result.reason}`);
+  }
 }
 
 async function main(argv = process.argv) {