feat(plan): merge plan-technical-review into /plan and split work into phases

[ryzizub]

Description

Two related pain points in the planning workflow:

1. Large plans cause AI to lose context during /build. Implementation drifts, conventions slip, and work degrades as the context window fills. plan-splitting-agent exists but only runs in /plan-technical-review and only splits across separate PRs (part-N files). It does not chunk work within a single plan for AI-sized execution.

2. /plan and /plan-technical-review fight each other. Review is an optional follow-up that gets skipped, so plan-splitting-agent rarely fires when it matters most. Plan quality checks belong inside plan creation, not as a separate skill the user has to remember.

Proposal: collapse the two skills into one and split work into phases by default during planning so /build can execute one phase per context window.

Requirements

1. Merge /plan-technical-review into /plan

• Move review agents (code-simplicity-review-agent, vgv-review-agent, plan-splitting-agent) into /plan as a mandatory step between writing the draft and presenting post-gen options.
• Run agents in parallel.
• Apply findings inline before the user sees the plan.
• Skip review for the minimal template (overkill for small plans).
• Delete /plan-technical-review skill, or keep a thin wrapper for reviewing externally-authored plans.

2. Integrate scope assessment into /plan

• plan-splitting-agent runs as part of the merged review step.
• If a split is recommended, present options: split into part-N files (existing behavior) or keep as a single plan with phased structure.
• Skip if the agent reports no split needed — small plans stay single-phase.

3. Add "Implementation Phases" structure to the standard template

Currently only extensive.md has phases. Standard should support phases too, with explicit shape per phase:

• Scope (1-2 sentences)
• Files touched
• Acceptance criteria
• Validation steps

Each phase should fit comfortably in one AI context window.

4. Update /build to loop phases, not just tasks

Current flow loops tasks linearly. Change to: load phase → implement → validate → commit → checkpoint → offer fresh context via /clear → next phase.

Reuse the existing "Clear context and [next step]" handoff pattern between phases.

Additional Context

• Plan skill: skills/plan/SKILL.md
• Plan technical review skill: skills/plan-technical-review/SKILL.md
• Plan splitting agent: agents/analysis/plan-splitting-agent.md
• Plan templates: skills/shared/references/plan-templates/
• Build skill: skills/build/SKILL.md
• Related: feat: add /validate or /evaluate phase #195 (validate phase), feat: number review findings #194 (number review findings)