fix: plan written to root PLAN.md instead of tasks/plan.md when flowing from /spec

[hiyochi]

Problem

When a user runs /spec and approves the generated SPEC.md, the gated workflow in spec-driven-development automatically transitions to Phase 2 (Plan). At this point it references planning-and-task-breakdown directly — bypassing the /plan command which contains the output path instructions.

The /plan command (.claude/commands/plan.md line 16) correctly states:

Save the plan to tasks/plan.md and task list to tasks/todo.md.

But this instruction exists only in .claude/commands/plan.md. The skill files themselves have no file path instructions. When the spec→plan transition happens within the gated workflow, the agent never sees the /plan command and writes PLAN.md to the project root.

Root Cause

File	Has output paths?
.claude/commands/plan.md	✅ tasks/plan.md + tasks/todo.md
skills/planning-and-task-breakdown/SKILL.md	❌ None
skills/spec-driven-development/SKILL.md Phase 2	❌ References skill directly, not command

spec-driven-development Phase 2 says "Follow planning-and-task-breakdown" — getting the template and process but NOT the file path convention.

Proposed Fix

Add explicit tasks/plan.md / tasks/todo.md output path instructions to:

1. planning-and-task-breakdown/SKILL.md — the canonical source for plan mechanics

• Step 1 (Enter Plan Mode): mention concrete output files
• New ## Output Files section before Plan Document Template

2. spec-driven-development/SKILL.md — Phase 2 callout

• Add **Output convention:** note so the path survives the spec→plan transition

PR ready at: #337