Improve server recovery and external agent workflows - #8

Summary

• keep browser sessions retrying across launcher shutdown and require the current access token after a new launcher starts
• isolate external-agent handoffs per terminal, prefer URL-first agentinfo discovery, and add standard and 3x mint actions to the status bar
• render background terminal PNGs from the xterm mirror canvas while retaining foreground DOM fidelity and rejecting degenerate 1x1 images
• update the published external-agent skill guidance and wrappers for multi-terminal operation

Validation

• tools/.venv_wsl/bin/python scripts/run_smoke_tests.py
• targeted browser smoke tests for token renewal, foreground PNG rendering, and background mirror-canvas PNG rendering
• live MIBCRK launcher restart and background term-2 capture: 2168x972 PNG, source terminal_mirror_canvas