feat(#44): add --diff and --output flags to snapshot command

[giulio-leone]

Summary

Adds incremental snapshot (--diff) and save-to-file (--output) options to reduce context consumption.

Changes

• --diff: Only returns lines that changed since the last snapshot (frequency-map comparison handles duplicate lines correctly)
• --output <path>: Saves snapshot to file instead of returning full content
• Previous snapshot captured before getSnapshot() to avoid race condition
• Fully typed response (no as any casts)

Usage

agent-browser snapshot --diff        # Only show changes since last snapshot
agent-browser snapshot -o snap.txt   # Save to file
agent-browser snapshot -i --diff     # Incremental interactive snapshot

Closes #44

[vercel]

@giulio-leone is attempting to deploy a commit to the Vercel Labs Team on Vercel.

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[ctate]

Thanks for the contribution, @giulio-leone! Reducing context consumption is a real need for AI agents. I pulled the branch, rebuilt both TypeScript and Rust CLI, and tested E2E.

What works:

• --diff returns (no changes) for identical snapshots
• --diff shows added/removed lines after navigation
• --output / -o saves snapshots to file

Issues to address:

1. --diff + --output combined -- --diff silently wins and the file is never written. Should either combine them or error.

2. --diff drops refs -- The diff code path early-returns without refs, so the agent can't interact with elements after a diff snapshot without a separate full snapshot. Undermines the token-saving goal.

3. Diff algorithm loses positional context -- The frequency-map approach groups all removals first, then all additions, with no indication of where in the tree changes occurred. A positional diff would be much more useful for agents reasoning about page structure.

4. Cross-mode lastSnapshot pollution -- lastSnapshot is shared daemon state regardless of snapshot options (-i, full, etc.). Alternating modes produces misleading diffs. Consider keying by options.

5. Missing documentation -- Per project guidelines, user-facing features need updates to cli/src/output.rs, README.md, skills/agent-browser/SKILL.md, and docs/src/app/snapshots/page.mdx. None were updated.