fix: prevent screenshot --annotate from hanging on complex pages

[juliobrasa]

Summary

On pages with many interactive elements, screenshot --annotate fires hundreds of boundingBox() calls simultaneously via Promise.all, saturating Chrome's CDP connection and causing timeouts/hangs (confirmed on both macOS and Linux).

Root cause

Object.entries(refs) on a complex page can yield 200+ entries. Calling locator.boundingBox() for all of them in a single Promise.all overwhelms the CDP websocket connection, causing Chrome to become unresponsive.

Fix

• Process elements in batches of 20 instead of all at once
• Add a 1-second timeout per element — elements that time out are skipped rather than blocking the entire operation
• No change to output format or annotation quality

Before

$ agent-browser screenshot --annotate  # complex page
# Hangs indefinitely, Chrome CPU spikes to 100%

After

$ agent-browser screenshot --annotate  # complex page
# Completes in seconds, some slow elements may be skipped

Test plan

• TypeScript compiles without errors
• Prettier passes (pre-commit hook)
• Manual test on simple page (example.com) — all annotations present
• Manual test on complex page (e.g. GitHub dashboard) — completes without hanging

Closes #509

🤖 Generated with Claude Code

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