[STG-2002] feat: migrate Convex Stagehand package

[shrey150]

Summary

• Migrates @browserbasehq/convex-stagehand into packages/convex-stagehand
• Adds packages/* to the pnpm workspace and root Changesets release scripts/config
• Adds a root release workflow for Changesets release PRs and npm publishing with GitHub OIDC
• Adds a patch changeset so the first release from this repo bumps from the already-published 0.1.0
• Adds vite as a direct dev dependency because the exported ./test helper references vite/client; npm flattened this transitively in the standalone repo, but pnpm release installs require it directly

Linear: https://linear.app/browserbase/issue/STG-2002/migrate-convex-stagehand-package-into-integrations

Release CI / npm publishing

This PR preserves the publishing path in-repo, but npm Trusted Publishing needs to be moved from browserbase/convex-stagehand to this repo before the publish job can succeed:

1. In npm, configure @browserbasehq/convex-stagehand Trusted Publisher to use repository browserbase/integrations.
2. Set the workflow filename field to release.yml (the file lives at .github/workflows/release.yml).
3. Leave environment unset unless npm currently requires one for this package.

Without that npm-side Trusted Publisher update, changeset publish should be able to create release PRs but the actual npm publish step is expected to fail authorization.

Validation

• pnpm install --no-frozen-lockfile
• pnpm run build:packages
• pnpm --dir packages/convex-stagehand run lint (passes with generated-file warnings)
• npm pack --dry-run in packages/convex-stagehand
• pnpm exec changeset status --verbose shows @browserbasehq/convex-stagehand -> 0.1.1

Notes:

• pnpm --dir packages/convex-stagehand run test fails the same way the standalone repo currently fails: Vitest reports missing @edge-runtime/vm and exits because no test files are present.
• Root pnpm run lint currently fails before reaching this package because examples/integrations/vercel/vercel-puppeteer/eslint.config.mjs imports @eslint/eslintrc, which is not a root dependency.