docs: add Forge product roadmap

[eshulman2]

Summary

• define Forge product direction and architectural boundaries
• sequence reliability, source-provider, Kubernetes, workflow-prototyping, and external-deployment work
• model mixed GitHub/GitLab changes as independent per-repository work items with aggregate workflow gates
• incorporate the current open-issue backlog, exit criteria, success measures, and product questions

Key decisions

• contracts precede new providers and execution backends
• Git providers and connections belong to each repository, not the workflow globally
• Kubernetes control-plane deployment and agent execution are separate tracks
• external deployment uses generic lifecycle hooks and plugins rather than vendor-specific infrastructure logic
• experimental workflows require dry-run, shadow evaluation, version pinning, and rollback

Validation

• git diff --check
• documentation-only change; MkDocs is not installed in the current development environment