Update workspace metadata for main default branches

[codeforester]

Context

Use main consistently across Base-family repositories and keep workspace metadata aligned with the actual GitHub defaults. The local base-workspace repo already tracks origin/main, but workspace.yaml still records default_branch: master for Base.

This should coordinate with basefoundry/base#904 and any other Base-family branch renames.

Scope

• Update workspace.yaml once base migrates from master to main.
• Check whether any other workspace entries need default-branch metadata refreshed after the branch-renaming pass.
• Prefer detected GitHub/default-branch behavior in future automation over hardcoded master metadata when possible.
• Update any workspace docs or validation notes if they describe old branch names.

Acceptance criteria

• workspace.yaml reflects main for every Base-family repo whose GitHub default branch is main.
• No workspace metadata points at master unless the corresponding repository still intentionally uses master.
• Workspace status/report output remains correct after the metadata update.