Configure OpenCode browser-harness to default to existing ULAA browser session with active visible tab #166

Summary

Configure the OpenCode browser-harness skill to strictly connect to the user's existing running ULAA browser (CDP at http://localhost:9222) and by default attach to/activate the currently active visible tab — matching BrowserCode's default behavior.

Current Behavior

• browser-harness requires explicit BU_CDP_URL=http://localhost:9222 env var
• new_tab() creates background tabs that aren't visually activated
• switch_tab() attaches CDP but doesn't call Target.activateTarget by default
• User must manually call activate_tab() to see actions

Desired Behavior (matching bcode)

• Default to existing ULAA browser on port 9222 (no env var needed)
• Automatically attach to current_tab() (the active visible tab)
• Automatically call Target.activateTarget so actions are visible
• new_tab() should activate the new tab by default
• Zero-config: works out of the box with running ULAA

Technical Approach

1. Modify ~/.config/opencode/skills/browser-harness/tool.py to inject BU_CDP_URL=http://localhost:9222 by default
2. Update skill to auto-attach to active tab on first command
3. Make activate_tab() the default for navigation actions
4. Test end-to-end on running ULAA instance

Acceptance Criteria

• No export BU_CDP_URL needed - works automatically
• First command attaches to currently active ULAA tab
• Navigation (goto_url) happens in the visible tab
• Click/type actions are visibly performed in ULAA
• new_tab() creates and activates a new visible tab