Use GitHub default_branch in pod spec create instead of hardcoded 'master'

[Copilot]

pod spec create was hardcoded to fall back to 'master' when looking up the default branch of a GitHub repo, causing failures for repos using 'main' (or any other non-master default branch).

Changes

• suggested_ref_and_version: Branch resolution now follows: repo['default_branch'] (current GitHub API field) → repo['master_branch'] (deprecated field) → 'main' (fallback)
• Renamed local variable master → default_branch_data to reflect it may not be master
• Updated error message: "master branch" → "default branch"

# Before
master_name = repo['master_branch'] || 'master'
master      = branches.find { |branch| branch['name'] == master_name }
raise Informative, "Unable to find any commits on the master branch ..."

# After
default_name        = repo['default_branch'] || repo['master_branch'] || 'main'
default_branch_data = branches.find { |branch| branch['name'] == default_name }
raise Informative, "Unable to find any commits on the default branch ..."

Original prompt

Main

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