v0.4.0

Agent Browser Core v0.4.0 release.

This release publishes the latest main, including Browser tabs and current shared host/UI contracts.

Local release checks completed on the merged main before tagging:

• npm run verify (395/395 tests)
• npm run verify:packages (5/5 tests)
• npm run verify:release-version -- v0.4.0
• npm run publish:packages:dry-run (7 packages)
• git diff --check