fix(execute): explain Node-side execute failures and keep extension builds loadable

[safzanpirani]

Summary

This PR fixes three failures that agents hit repeatedly while using Browser Control.

• Page globals in Node-side execute code. Execute code runs in Node, and agents often write it as page code. window.location.href fails with ReferenceError: window is not defined, and fetch("/api/me") fails with TypeError: Failed to parse URL from /api/me, with no hint about the cause. Execute now adds a warning for both errors that points to page.evaluate. The Execute Interface section of the skill now states that execute code runs in Node.
• execute --file errors without a reason. A missing or unreadable file produced only read execute file /tmp/chat_.js. The message now includes the file-system reason, as doctor already does for its read errors. For example, read execute file /tmp/chat_.js: NotFound: FileSystem.readFile (/tmp/chat_.js).
• pnpm build:extension deleted extension/dist before building. If esbuild failed, the unpacked extension directory stayed empty and Chromium could not load it on the next worker start. The extension build now writes into a staging directory under the OS temp directory. It then copies each file into extension/dist, renames it into place, and removes stale entries. The directory and manifest.json stay in place during the build, and a failed build leaves the previous output untouched. --outdir keeps its existing fresh-directory rules, and build:cli is unchanged.

Verification

• pnpm run ci, the runtime:prepare step from CI, and pnpm typecheck pass locally. pnpm test passes 971 tests in 69 files, and pnpm audit:duplicates finds 0 clones.
• I did not run the browser smoke set.
• A new unit test in test/execute-ergonomics.test.ts covers the window and relative fetch warnings. It also checks that an unrelated ReferenceError and an in-page page.evaluate error get no warning.
• A second pnpm build:extension kept the same extension/dist inode and removed a stale file. A build with a syntax error in background.ts exited 1 and left manifest.json and the previous background.js byte-identical. No temp files remained in dist or the staging location.
• execute --file with a missing path prints the reason shown above, before any relay contact.