ci: add automated release workflows for pub.dev publishing

[spydon]

Summary

• Adds a three-workflow release pipeline modeled on supabase-flutter's release process:
  • release-prepare.yml (workflow_dispatch): bumps versions/changelogs via Melos versioning and opens a chore(release): publish packages PR (supports prerelease and graduate inputs).
  • release-tag.yml (push to main on a chore(release): commit): tags each changed package and triggers release-publish.yml per package.
  • release-publish.yml (workflow_dispatch, triggered per tag): runs melos publish via pub.dev trusted publishing and creates the GitHub release.
• Updates CONTRIBUTING.md §7 to describe the new automated release process.

Prerequisites before this can run end to end

• GITHUB_TOKEN is used throughout (no GitHub App). If org/repo settings block Actions from opening PRs or pushing tags on main, enable "Allow GitHub Actions to create and approve pull requests" or switch to a PAT/App token.
• pub.dev trusted publishing (OIDC) must be configured per package (Admin → Automated publishing → GitHub Actions) for passkeys, passkeys_android, passkeys_darwin, passkeys_web, passkeys_windows, passkeys_platform_interface, corbado_auth, and corbado_auth_firebase before release-publish.yml can publish.

Test plan

• Validate YAML syntax (done locally via Ruby's YAML parser)
• Configure pub.dev trusted publishing for each package
• Dry-run release-prepare.yml on a branch with pending changes and confirm the PR body/versions look right
• Merge the release PR and confirm release-tag.yml tags the right packages and triggers release-publish.yml only for changed ones
• Confirm release-publish.yml publishes to pub.dev and creates a GitHub release