Enable staging tag deployments: Feature #128

[fairybladderwort]

Feature

Feature #128
Implements:

• Implement trusted staging-tag deployment routing and preserve the main compare-and-swap path. #129

Current task commit: f3548f6

Stack base: main
Task dependencies: None

Validation:

• RBENV_VERSION=4.0.5 rbenv exec bundle exec rspec spec/deployment (92 examples, 0 failures)
• Workflow YAML parse
• git diff --check

Remaining tasks:

• Add and test the no-argument bin/bump_staging_tag helper. #130: staging tag helper
• Document and validate the staging authorization and operations contract. #131: operations documentation