docs: add obol stack release train playbook

[bussyjd]

Summary

• Adds a release-train.md reference to the obol-stack development skill.
• Routes PR train, ordered merge/collapse, frontend RC, Renovate/GHAS, running-cluster upgrade, and RC release work to that reference.
• Keeps host-specific QA LLM endpoints and credentials out of the reusable workflow.

Why

Recent work repeatedly combined PR range review, architectural correctness checks, fix PRs, bot/security review comments, image pin verification, release template discipline, smoke validation, and scoped cleanup. The existing obol-stack skill covered the individual smoke and QA pieces, but not the orchestration of a full PR/release train.

flowchart LR
    A["Inventory PRs and checks"] --> B["Architectural review"]
    B --> C{"Incorrect or risky?"}
    C -- "yes" --> D["Open fix/<topic> PR"]
    C -- "no" --> E["Mark ready / merge in order"]
    D --> F["Parallel targeted validation"]
    F --> E
    E --> G["Upgrade running cluster"]
    G --> H["Release-smoke gate"]
    H --> I{"Green enough to release?"}
    I -- "no" --> J["Record blockers"]
    I -- "yes" --> K["Template-based non-draft RC release"]

Loading

Validation

• git diff --cached --check
• python3 /Users/bussyjd/.codex/skills/.system/skill-creator/scripts/quick_validate.py /Users/bussyjd/.codex/worktrees/a251/obol-stack/.agents/skills/obol-stack-dev