Support non-main default branch on repo creation

[dev-milos]

Problem

When a YAML config for a new repository sets default_branch to anything other than main, Terraform fails at apply (issue #14).

auto_init creates the repo with a single main branch, then github_branch_default tries to point the default at e.g. master, which does not exist yet:

Error: PATCH /repos/<org>/<repo>: 422 Validation Failed
  Field: default_branch  Code: invalid
  Message: The branch master was not found. Please push that ref first or create it via the Git Data API.

The repo is left created but with the wrong default branch.

Fix

Set rename = true on github_branch_default so the provider renames the auto-init branch (main) to the requested default instead of expecting it to already exist.

Renaming is applied only when needed:

rename = local.default_branch != "main" && !contains(keys(local.branches_map), local.default_branch)

• target is main → no rename (it's already the auto-init branch)
• target is an explicitly managed github_branch → no rename (that branch is created separately)
• otherwise (e.g. master) → rename the auto-init branch to the target

rename is wrapped in lifecycle { ignore_changes = [rename] } so existing and imported repos whose default already matches do not show a perpetual diff.

Testing

Reproduced and verified against a test org via local terraform apply:

• Before: new repo with default_branch: master → repo created, apply errors with the 422 above; repo left with default main.
• After: same config → apply succeeds, repo default branch is master, sole branch renamed from main to master.

Closes #14