docs: add Coder SSH-tunnel guide for reaching adk web

[ethancjackson]

Summary

• Add guide 5: connect a local browser to adk web running inside a Coder workspace (macOS / Windows / Linux, including coder port-forward as the simple path).
• Keep it off Path A/B — it is Coder environment setup, not a forecasting worksheet — and index it from the Coder README section, the guides table, the atlas chooser, and the existing localhost:8000 / adk web call sites.

Test plan

• Open guides/05-access-adk-web-via-ssh-tunnel.md and walk the steps from a laptop against a running Coder workspace (uv run adk web … still up in the workspace terminal).
• Confirm Option B (coder port-forward <name> --tcp 8000:8000) serves http://localhost:8000.
• Confirm the Option A vs B name split: SSH host is coder.<dashboard-name>, port-forward takes the dashboard name with no prefix.
• Click through from README → Coder Workspaces, guides/README.md row 5, getting-started README / concierge notebook, energy notebooks 05–06, and the atlas §06 chooser row.

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