feat(cli): add --tab flag for tab-scoped commands

[mvanhorn]

Summary

Add a --tab N global flag that runs a command against a specific tab without switching the active tab. Useful for multi-tab scraping workflows where you need to interact with background tabs without losing your current context.

# Take screenshot of tab 2 without switching to it
agent-browser --tab 2 screenshot

# Get the URL of tab 1
agent-browser --tab 1 url

# Click an element in tab 0
agent-browser --tab 0 click @e1

Related: #853

Evidence

Signal	Source
Issue #853	"SPA Scraping Pattern: Preserving accessibility refs with tab isolation"
Issue #74	Tab management friction (tab list didn't show all tabs)
Gap	Must currently switch tabs to run commands, losing active tab context
Reddit r/ClaudeAI	"Whats your reliable browser setup for Claude Code?" - parallel tab management is a pain point
Reddit r/AI_Agents	"Built a fully autonomous system to coordinate 100+ browser automation agents"
NxCode	"Parallel browser session management" identified as universal unsolved gap

Implementation

The --tab flag is parsed in flags.rs and injected as tabIndex into the command JSON in main.rs. The daemon's handle_action in actions.rs temporarily overrides the active_page_index on BrowserManager for the duration of the command, then restores the original index afterward. This means all existing handlers work with --tab without modification.

Files changed

• cli/src/flags.rs - parse --tab N flag, add to clean_args skip list
• cli/src/main.rs - inject tabIndex into command JSON
• cli/src/native/actions.rs - temporarily swap active page index before/after dispatch
• cli/src/native/browser.rs - add active_page() and set_active_page() methods
• cli/src/commands.rs - add tab_index to test default_flags
• cli/src/output.rs - add --tab to help text

Test plan

• cargo fmt passes
• cargo clippy passes
• cargo test passes (473 tests, 0 failures)
• Manual: agent-browser --tab 1 url returns URL of tab 1 without switching active tab
• Manual: agent-browser --tab 0 screenshot takes screenshot of tab 0

This contribution was developed with AI assistance (Claude Code).

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