chore: remove legacy release tooling and document new flow

Deletes the old release_gem.yml workflow, trigger scripts, and RELEASING.md.
Updates DEVELOPING.md with a Releasing section describing the new
draft PR → merge → tag → publish flow.

Assisted-by: Claude Code:claude-sonnet-4-6

Author: JP-Ellis

.github/workflows/release_gem.yml

@@ -15,3 +15,19 @@
 ## Generating docs
 
 If you change the CLI commands, run `bundle exec script/update-cli-usage-in-readme.rb` to update the `README.md` automatically.
+
+## Releasing
+
+Releases are fully automated via the PR-driven flow in `.github/workflows/release.yml`.
+
+1. **On every push to `master`**, the `prepare` job runs `scripts/release.rb prepare`, which uses git-cliff to compute the next semver, updates `lib/pact_broker/client/version.rb` and `CHANGELOG.md`, force-pushes the changes to the `release/pact-broker-client` branch, and creates or updates a **draft** release PR.
+
+2. **When ready to release**, promote the PR from draft → ready-for-review. This triggers CI. You can edit `version.rb` or `CHANGELOG.md` directly on the `release/pact-broker-client` branch to make manual adjustments — those files are the source of truth.
+
+3. **Merging the PR** triggers the `tag` job, which reads the version from `version.rb` and pushes a `v{version}` tag.
+
+4. **The tag push** triggers the `publish` job: runs tests, builds the gem, creates a GitHub Release, and pushes to RubyGems via OIDC (no stored API key required).
+
+### One-time setup (already done once per gem)
+
+Configure OIDC trusted publishing on [rubygems.org](https://rubygems.org) for the `pact_broker-client` gem, trusting the `publish` job in `pact-foundation/pact_broker-client`.