Adopt main as Base's default branch

[codeforester]

Context

Use main as the default branch consistently across Base-managed repositories going forward. The local base checkout currently tracks origin/master, and a scan found master assumptions across user-facing docs, bootstrap/install URLs, repo guidance, and basectl default-branch tests/fallbacks.

Examples found locally include README.md, FAQ.md, docs/bootstrap.md, docs/release-process.md, docs/github-workflow.md, AGENTS.md, CONTRIBUTING.md, skills.md, templates/project-install.sh, and basectl tests/helpers.

Scope

• Rename the repository default branch from master to main when the compatibility plan is ready.
• Update branch protection/rulesets and any workflow settings that explicitly target master.
• Replace hardcoded origin/master, --base master, and raw GitHub /master/ install URLs with main, the detected default branch, or a release/tagged URL where that is the safer contract.
• Keep basectl gh, basectl repo, and basectl update main-first while preserving support for existing repositories that still use master.
• Update canonical docs, generated guidance, templates, and any repo context files that document the branch workflow.
• Document the post-rename local clone update commands and compatibility expectations for contributors.

Acceptance criteria

• GitHub reports main as the default branch for base.
• Non-historical docs/templates no longer instruct new work to branch from origin/master or open PRs against master.
• Bootstrap/install examples remain valid after the rename.
• Tests cover main as the normal default and master as a supported legacy fallback.
• Branch protections, release workflow, issue/PR workflow, and cleanup commands are verified after the rename.

Related follow-up

Coordinate this with the remaining Base-family repos that still reference or depend on master: base-bash-libs, homebrew-base, base-workspace, and base-demo.

[codeforester]

Follow-up issues created for the cross-repo branch-name cleanup:

• Adopt main as base-bash-libs default branch base-bash-libs#46 - adopt main as the shell library default branch and keep git helpers main-first with master fallback coverage.
• Update Homebrew tap for Base-family main branches homebrew-base#50 - update tap default branch, formula head branches, bottle workflow assumptions, and tap docs.
• basefoundry/base-workspace#3 - refresh workspace metadata after repo branch renames.
• Update Base installer reference for main branch migration base-demo#102 - update the demo installer reference after Base moves from master to main.