Update Base installer reference for main branch migration

[codeforester]

Context

Use main consistently across Base-family repositories. The local base-demo repo already tracks origin/main, but install.sh still defaults BASE_INSTALL_URL to https://raw.githubusercontent.com/basefoundry/base/master/install.sh.

This should coordinate with basefoundry/base#904.

Scope

• Update the default Base installer URL after the Base repository migrates from master to main, or switch to a tag/release URL if that is the safer demo contract.
• Check docs and walkthroughs for any live master branch references. Historical changelog entries do not need churn unless they affect current instructions.
• Verify the demo install path still works after the Base branch rename.

Acceptance criteria

• Current demo installer code no longer depends on Base master once Base uses main.
• Demo documentation and walkthroughs use main, the detected default branch, or a stable release reference consistently.
• The representative demo install path is validated after the update.