setup skill: offer pr_body_required_sections in interview/write template

[kyle-sexton]

Problem

#975 (PR #1029) added the pr_body_required_sections config key on .claude/source-control.md,
resolved by /pull-request create and documented in plugins/source-control/reference/config-resolution.md.
The fleet's sanctioned adoption path for every other key on that surface is /source-control:setup —
check reports the effective value/source, apply interviews and writes it. pr_body_required_sections
has neither: check doesn't report it and apply's interview/write template doesn't offer it, so a
consumer's only path to configuring the key is hand-editing .claude/source-control.md directly. That
stalls rollout — the key exists but the sanctioned onboarding flow doesn't know about it.

Scope

Extend plugins/source-control/skills/setup/SKILL.md to bring pr_body_required_sections to parity
with the other per-key surfaces (subject_pattern, pr_title_pattern, trailer_policy,
pr_body_attribution):

• check reports the key's effective value and which layer supplied it (or the portable default when
  unset).
• apply's interview offers setting it, recommending the plugin's own portable default
  (Summary, Test plan) rather than any specific org's convention — the setup interview must stay
  agnostic, matching the plugin's Two-lane convention posture.
• The write template (the .claude/source-control.md section list apply materializes) gains the new
  ## pr_body_required_sections section, following the same "omit when unset" rule the other optional
  keys already follow.

Refs #975
Refs #1029