Automate reviewable development docs sync PRs after stack releases

Problem

Each stack release publishes an owned inventory, but updating the development
version catalog still requires a maintainer to run
docs-version-sync --update-catalog and open a Pull Request manually.

Top-of-tree documentation can drift after stack releases even when all three
inventory assets are available.

Desired behavior

After stack inventories are published, automation opens or updates one
reviewable development documentation Pull Request. QA qualification and stable
documentation cuts remain explicit maintainer actions.

Scope

• Trigger after a stack inventory release, with a manual retry option.
• Wait until complete inventory assets are available for all three stacks.
• Coalesce closely timed stack releases into one development update.
• Run the catalog update, generated documentation update, and offline checks.
• Open or update one bot-owned Pull Request with the exact three stack versions.
• Keep synchronization failures advisory during the initial rollout.

Acceptance criteria

• A completed three-stack release set creates or updates one reviewable Pull
  Request.
• The Pull Request records the selected control-plane, compute-plane, and
  observability versions.
• No Pull Request is opened when generated content is unchanged.
• Repeated runs are idempotent and reuse the existing automation branch.
• A missing or invalid inventory produces an actionable warning and does not
  modify stable documentation.
• The workflow never qualifies a release set or cuts versioned documentation.
• Logs and generated changes contain no credentials or private registry data.

References

• Relates to #1838
• Follow-up to #1839

Start by locating the stack inventory publication flow and the docs-version-sync --update-catalog entry point; the issue names no files or tests. Trace how the three inventory assets and generated documentation checks are currently run, then verify that the resulting automation meets the acceptance criteria without qualifying releases or changing stable documentation.