feat(writing-plans): add parallel suitability check to plan handoff

[leonsong09]

Summary

This is a narrow writing-plans improvement aimed at making execution routing more explicit without making Superpowers parallel-first.

It adds:

• per-task execution metadata in the plan template:
  • Depends on
  • Write Scope
  • Verify
  • Potential Conflicts
• a short Parallel Suitability Check section before execution handoff
• a handoff pattern that gives a recommended execution mode (Iterative or Parallel-safe) with reasoning
• plan-reviewer guidance to check whether the plan is explicit enough to choose the right execution path
• small prompt fixture updates so the explicit-skill-request prompts reflect the new handoff wording

Why

Today writing-plans decomposes work into tasks, but the handoff does not make it easy to tell:

• which tasks are truly independent
• which tasks share write scope or contracts
• whether the safest next step is iterative execution or a parallel-safe path

This PR keeps the current correctness-first bias, but makes plans clearer and more actionable when some tasks are independent.

Non-goals

This PR does not:

• make Superpowers parallel-by-default
• introduce a new orchestration skill
• replace heavier multi-agent flows like agent-teams / assemble
• change the recommendation that iterative execution is safest when tasks are coupled

Related discussion

• Refs Multi-agent collaboration to complete tasks #777
• Related to New skill: agent-teams — full plan → team → synthesize workflow #801
• Consistent with the correctness-first preference discussed in Parallel subagents do not start automatically #945

Verification

Ran targeted verification directly related to these documentation/prompt changes:

• git diff --check
• grep/rg checks for the new handoff markers (Execution Recommendation, Parallel Suitability Check, Execution Readiness)
• confirmed changed file set stayed limited to the intended 4 files

I did not claim broader build/test coverage because this change is limited to skill/prompt markdown and there is no direct unit/build target for it in this repo.