Demonstrate basectl docs in demo walkthrough

[codeforester]

Context

basectl docs (added in Base v1.3.0) opens the Base documentation home page. In non-interactive or CI contexts, basectl docs --show-url prints the URL cleanly without opening a browser. The command is part of the documented Base command surface but is not exercised anywhere in the base-demo demo walkthrough (15 steps) or CI validation.

The demo walkthrough is the canonical teaching surface for the Base command set. Omitting basectl docs means learners discover it only through basectl --help rather than through the narrative flow of the demo.

Scope

• Add a short step to demo/demo.sh — after the existing observability or export-context step — that runs basectl docs --show-url and asserts the output contains github.com/basefoundry/base.
• Use capture_command so the URL appears in the demo output.
• Add a corresponding CI step in tests.yml that runs basectl docs --show-url.
• Add basectl docs --show-url to the Quick Start command list in README.md.

Validation

• demo/demo.sh --non-interactive includes the basectl docs --show-url step and passes.
• The CI validate job includes the basectl docs --show-url step.
• README.md lists the command.