Migrate and document the planning pull request workflow

[rochecompaan]

Part of #180.

Blocked by: #189

Goal

Complete compatibility behavior, user guidance, and end-to-end verification for the planning pull request workflow.

Scope

• Add deprecation behavior for legacy artifact setters and plan-only routing.
• Add the skill-pack updater notice.
• Update domain language, installed planning guidance, and user documentation.
• Add full GitHub and Forgejo gate-matrix, interruption, and recovery scenarios.
• Run the final targeted and repository verification commands.

Acceptance criteria

• Deprecated paths explain the replacement workflow and remain deterministic.
• Skill-pack config, metadata, installed files, and version references agree.
• Documentation describes review gates, recovery, and manual stale-lock handling.
• Full scenario tests cover all gate combinations and both providers.
• Required build, test, lint, site, and Nix verification passes.

[rochecompaan]

This was generated by AI during triage.

Triage Notes

This work is clear and suitable for automation, but it cannot start until the prerequisite integration work is complete.

Blocked by: #189

Re-evaluate for agent-ready after #189 closes.