Change default branch name from 'master' to 'main' for GitHub imports…

[KiserDesigns]

… and GitHub-linked projects.

The GitHub-wide default 'master' -> 'main' transition was years ago. New projects should be using 'main'.

Signed-off by: Noah Kiser

[emindeniz99]

Related work heads-up: #77 (just marked ready) touches the same import-default problem from a different angle — instead of flipping the hardcoded master to main, it removes the hardcode entirely and lets an empty branch resolve to the repository's actual default branch via codeload HEAD, so both main-default and legacy master-default repositories import correctly (most classic Pebble repos still default to master). The GitHub settings pane half of this PR is untouched by #77 (declared there as a follow-up) — the same no-hardcode treatment would work nicely there too. Flagging so the maintainers can decide how to sequence the two.

Co-Authored-By: Claude Fable 5 noreply@anthropic.com