fix(ship): link issues by intent at publish and handoff; Fixes: trailer at commit (Lane D: D1, D2, D-a)

- Step 15: a commit fixing a user-named issue carries a `Fixes: #<n>` trailer
  (offset by tightening Step 15 so the skeleton stays at 80,998 <= 81,000).
- pr-body: an explicit "Issue links" step runs gstack-issue-links before the
  body is composed (previously only a placeholder inside the body template);
  interactive runs ask once about `(promote: fix #N)` references and add
  `--supersedes`/`--credit` only on the owner's confirmation; unattended runs
  keep Refs and list unpromoted candidates, Related lines and lookup failures
  in the completion message; `ISSUE_LINKS_MISSING` from the receipt write.
- handoff (no-PR lanes): runs gstack-issue-links and passes --issues to the
  receipt; the completion message gains an `issues:` line.
- Regenerated ship/SKILL.md and sections; refreshed the four ship goldens.
- Parity: union 309,431 / 187,706 = 1.6485 (> 1.6450 by 655 bytes); maxSizeRatio
  raised to the measured 1.6485 with its justification, as previous waves did.

Files: ship/SKILL.md.tmpl, ship/SKILL.md, ship/sections/pr-body.md.tmpl,
ship/sections/pr-body.md, ship/sections/handoff.md.tmpl, ship/sections/handoff.md,
test/fixtures/golden/{claude,codex,factory,agy}-ship-SKILL.md,
test/helpers/carve-guards.ts (ship maxSizeRatio).

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

docs/pregate.md

@@ -363,9 +363,10 @@ every refused match is printed with its reason.
 
 Open PRs by other authors (not the `gh` user, not this branch) whose closing
 set overlaps this PR's `Fixes` are `Related: #N (@author)`, read page by page
-from each issue's closing-PR references, in unattended runs too. `Supersedes #N
-(@author)` replaces that line and `Contributed by @author (#N)` is added only
-with `--supersedes N` / `--credit N`, after the owner confirms.
+from each issue's closing-PR references, in unattended runs too. `Supersedes #N`
+replaces that line and `Contributed by @author` is added (the credit lines of
+`lib/contributor-mode.ts`) only with `--supersedes N` / `--credit N`, after the
+owner confirms.
 `--no-auto-links` skips the lookup. A failed lookup prints `ISSUE_LOOKUP_FAILED`
 and keeps the other lines. The stderr preview names the source of each line.