Fix Bootstrap: replace gh pr create with REST API + branch propagation polling

[Copilot]

gh pr create --repo Cratis/$repo --head "$branch" fails with Head sha can't be blank, Base sha can't be blank, Head ref must be a branch on every repo because gh resolves --head against the local checkout (Cratis/Workflows), not the target repo.

Changes

• Replace gh pr create with gh api -X POST repos/Cratis/$repo/pulls — the REST endpoint resolves the head branch in the target repo's context, no local git lookup involved. Uses plain branch name (correct for same-repo, non-fork PRs).

• Replace fixed sleep 10 with branch propagation polling — polls GET /repos/Cratis/$repo/branches/$branch (same replica as the PR creation service) up to 6 × 10s before attempting PR creation. Skips PR creation with a warning if the branch isn't visible after 60s.

• Reduce max_attempts from 4 → 3 — smarter pre-flight check makes excessive blind retries unnecessary.

# Before — resolves head branch from local Cratis/Workflows checkout → always blank
gh pr create --repo "Cratis/$repo" --head "$branch" --base "$default_branch"

# After — resolves head branch in target repo context via REST
gh api -X POST "repos/Cratis/$repo/pulls" \
  -f head="$branch" \
  -f base="$default_branch" \
  ...

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