Consider removing outdated specs/12-project-plan.md and associated references

[zacateras]

The file specs/12-project-plan.md and its contents appear to be outdated and no longer maintained. Consider removing this project planning file to avoid confusion and reduce outdated references in the repository. If useful, link any related specs or docs that are still relevant in a new or updated planning/roadmap reference.

References:

• Outdated stream and execution order information in specs/12-project-plan.md
• Duplicate or superseded coverage by more current specs, docs, or project boards

Acceptance Criteria:

• specs/12-project-plan.md is deleted if determined fully obsolete
• Any uniquely relevant content (if any) is migrated to a current place
• Documentation no longer references 12-project-plan.md or its outdated status