Add release plan payloads for workflow automation

[mavam]

🔍 Problem

• Release automation could not inspect the next release in a structured way.
• The workflow-dispatch intro had to be authored manually, even when the pending changelog entries already contained enough context.
• Release planning and release creation shared logic implicitly, which made it harder to guarantee that previews matched the actual release snapshot.

🛠️ Solution

• Add tenzir-ship release plan, including --json, to expose the resolved release snapshot for automation.
• Reuse the same snapshot-planning logic in both release plan and release create.
• Add a workflow helper script that derives a release intro from the JSON plan when the dispatch input is left empty.
• Update the repo release trigger workflow to prepare metadata before invoking the reusable release workflow.
• Cover the new CLI and workflow behavior with tests and document the automation flow.

💬 Review

• Focus on the shared snapshot-planning path in src/tenzir_ship/cli/_release.py and whether it preserves current release/create semantics.
• Check the shape and stability of the JSON payload for downstream automation.
• Check the workflow fallback expression and generated intro behavior for both normal releases and RC promotion.