docs: add dual-instance planning pattern (Jon Williams)

Add vertical separation pattern (planner/implementer) as complement to horizontal scaling (Boris pattern).

## Changes

**Main guide (ultimate-guide.md)**:
- New Section 9.17.1: "Alternative Pattern: Dual-Instance Planning" (~350 lines)
  - When to use (solo devs, spec-heavy, $100-200/month)
  - Setup instructions (2 Claude instances, Plans/ directory)
  - Complete workflow (5 phases: planning, review, implementation, verification, archive)
  - Comparison table (Boris horizontal vs Jon vertical scaling)
  - Cost analysis (2 instances vs correction loops)
  - Agent-ready plan best practices
  - Limitations and tips

**Workflow file (workflows/dual-instance-planning.md)**:
- Full workflow guide (~750 lines)
- Complete example (JWT auth implementation)
- Plan template (ready to copy-paste)
- Cost breakdown and decision matrix
- Troubleshooting and bash aliases

**References updated**:
- machine-readable/reference.yaml: 15 new entries
  - dual_instance_planning, dual_instance_workflow, etc.
  - Line numbers, source attribution, metadata
- guide/workflows/plan-driven.md: Link in See Also section
- README.md: Update evaluation count (46 → 47)

**Evaluation documented**:
- docs/resource-evaluations/jon-williams-dual-instance-pattern.md
  - Full methodology (fetch, analyze, challenge, fact-check)
  - Score progression (2-3/5 → 4/5 after technical-writer challenge)
  - Gap analysis, comparison, integration rationale

## Source
LinkedIn post by Jon Williams (Product Designer, UK)
Date: 2026-02-03
URL: https://www.linkedin.com/posts/thatjonwilliams_ive-been-using-cursor-for-six-months-now-activity-7424481861802033153-k8bu

Context: Transition from Cursor (6 months) to Claude Code with Opus 4.5
Pattern: Vertical separation (Claude Zero: planning/review, Claude One: implementation)
Distinction: Orthogonal to Boris pattern (vertical vs horizontal scaling)

## Stats
- Lines added: ~1,400
- Files modified: 4
- Files created: 2 (workflow + evaluation)
- References added: 15 (reference.yaml)
- Evaluation score: 4/5 (High Value)
- Integration time: ~2.5 hours

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

guide/workflows/plan-driven.md

@@ -250,3 +250,4 @@ Plans in `.claude/plans/` serve as decision documentation:
 - [spec-first.md](./spec-first.md) — Combine with Spec-First
 - [iterative-refinement.md](./iterative-refinement.md) — Post-plan iteration
 - [task-management.md](./task-management.md) — Track plan execution across sessions with Tasks API
+- [dual-instance-planning.md](./dual-instance-planning.md) — Advanced: Use two Claude instances (planner + implementer) for quality-focused workflows