#2456 Remove stale Stagehand V4 package references

Summary

• update the V4 Python landing-page install command from stagehand-v4 to stagehand
• update the Python AI-rules prompt to use the published stagehand distribution name
• remove the obsolete v4 suffix from the Browserbase smoke-test session label

Why

PR #2400 renamed the V4 Python distribution to stagehand, but two restored docs references still used the temporary stagehand-v4 name after PR #2428 merged. A repository-wide audit found no other hyphenated stagehand-v4 references after this change. The remaining stagehand_v4 strings in the Python build helper and its test are intentionally retained so old pre-rename artifacts are cleaned from dist/.

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Summary by cubic

Replace remaining stagehand-v4 references with stagehand in V4 docs and Browserbase smoke test metadata. This fixes the Python install command, aligns the AI-rules prompt with the published package name, and removes the obsolete -v4 suffix from the test label.