feat: add interactive demo workspace bootstrap

[magix022]

Rationale

The web-operator change needs a directly runnable follow-up bootstrapper so users can create a local Avalanche workspace with editable framework checkouts and start the browser UI without relying on the retired demo-init skill flow.

Summary

• Add an interactive root-level init.sh bootstrapper for an empty workspace.
• Clone Avalanche and PredictRLM as editable local checkouts; copy the Avalanche authoring skill into project scope.
• Provide current provider/model selections and a deferred Other-provider path that records exactly where to set lm and the provider API key later.
• Generate handoff guidance that starts ava operator --web for the browser UI.
• Update bootstrap documentation and remove superseded OpenSpec planning artifacts.

Follow-up to #39.

Test Plan

• bash -n init.sh
• Complete interactive bootstrap with a custom-provider selection; verified generated flow, .env, and workspace guidance.
• Complete interactive bootstrap with the deferred Other-provider selection; verified no model/key prompt and explicit generated configuration guidance.
• uv run pytest test/operator_tests/test_web.py test/cli_test.py -q

[ereinach]

approved