feat(brief): add PR-body Intent contract to PR-producing ship briefs

[Bre77]

What

• Crewmate briefs carried no durable rule for PR-body shape, so auto-generated bodies could open with prose walls or the raw task prompt at the top.
  • Fix: the ship-brief scaffold (bin/fm-brief.sh) now emits a # PR body contract section for both PR-producing modes (no-mistakes and direct-PR).
  • The crewmate must make the opening ## Intent / ## What section a scannable tree (typically 3-8 top-level bullets, nested 2-3 levels), restructuring with gh-axi pr edit when the auto-generated body needs it.
  • Top-level bullets name the issues addressed; sub-bullets carry fix approach, implications, and explicit scope boundaries; the shape flexes by change type rather than following a rigid template.
  • Long-form narrative or the original prompt may sit only at the bottom of the body inside a collapsed <details> block.
  • Repos with their own strict upstream PR template win: the crewmate follows that template with a concise, human description instead.
• AGENTS.md section 11 gains one pointer sentence naming the scaffold as the contract owner (one-owner rule; the rule text is not duplicated there).
• Scope: local-only briefs deliberately get no PR-body section (no PR exists), and scout/secondmate briefs are untouched.

Testing

• bash tests/fm-brief.test.sh - 4/4 pass, including the new test_pr_body_contract_by_mode (section present for no-mistakes and direct-PR, absent for local-only).
• shellcheck bin/*.sh bin/backends/*.sh tests/*.sh (v0.10.0) - clean, invoked exactly as CI does.
• The new block reuses the $(cat <<EOF) pattern with no apostrophes, so the existing bash -n regression guard for issue firstmate's fm-brief.sh scaffold script has a bug #166 covers it.

Full narrative / original brief

Captain's rule (2026-07-06): the PR body's opening section (## Intent, or ## What on direct-PR bodies) must open with a scannable tree, not prose walls - top-level bullets for the issues addressed, sub-bullets for fix approach, implications, and scope boundaries, typically 3-8 top-level bullets nested 2-3 levels max.
The structure flexes with the change type (bug fix: issue -> root cause -> fix -> implication; feature: goal -> approach -> limits; ops: what -> why -> blast radius).
Long-form narrative and/or the original task prompt are permitted but must sit at the bottom of the body inside a collapsed details block, never at the top.
All other sections (What Changed, Risk Assessment, Testing, evidence details, post-deploy ops notes) are preserved as-is.
Captain's addendum: the tree-Intent contract applies only where we own the PR template; repos with their own strict upstream PR template follow that template instead, and those descriptions stay concise and human - explain why and toward what goal, not a line-by-line tour of the diff.

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