docs: add Forge product roadmap

[eshulman2]

Summary

• define Forge product direction, architectural boundaries, sequencing, and resolved product decisions
• prioritize enterprise platform teams operating Forge centrally while preserving open-source self-hosting, extensibility, and contribution
• require mixed GitHub and multiple-GitLab-instance workflows from the first GitLab milestone
• cover both Kubernetes control-plane deployment and isolated agent execution
• add PRD-driven product discovery through competing, revisable prototypes whose approved learnings feed back into planning
• add security remediation, generated-code scanning, Forge hardening, verification-debt testing, dynamic model routing, and optional MLflow evaluation
• map every roadmap theme to relevant open issues, active PRs, merged foundations, and explicit tracking gaps as of 2026-08-10

Key decisions

• workflow logic remains provider-neutral behind versioned domain interfaces and conformance tests
• source and issue-tracker integrations support Forge-maintained adapters and external plugins
• Kubernetes control-plane deployment and agent execution are separate shippable tracks
• preview environments require explicit approval by default
• the first external deployment integration invokes an allowlisted automation job
• experimental workflow definitions require dry-run, shadow evaluation, version pinning, and rollback
• Jira remains the built-in default but is not a permanent architectural requirement

Validation

• git diff --check
• repository issue and PR history audited through 2026-08-10
• documentation-only change