Produce an implementation-ready plan and optionally begin coding based on all prior pipeline outputs.
Use as step 7 (final step) of the product pipeline, after the sprint plan is complete. Requires
`06-sprint-plan.md` and all prior outputs. Produces an implementation plan with file structure,
coding standards, and initial setup instructions.
Examples:
- user: "Create implementation plan" → produce dev-ready plan
- user: "Step 7 of the pipeline" → load sprint plan and create implementation plan
- user: "Start coding" → produce plan and begin implementation
