fix(issue-links): --author for the Related lookup when gh runs as an app (Lane D: D2)

On a Capy machine the gh viewer is `capy-ai[bot]`, not the owner (checked live
against garrytan/gbrain #6252), so "other authors" needs the PR author named:
`--author <login>` overrides the viewer; the pr-body Issue links step says to
pass it when gh runs as an app.

Files: bin/gstack-issue-links, docs/pregate.md, ship/sections/pr-body.md.tmpl,
ship/sections/pr-body.md, test/ship-issue-intent.test.ts,
test/helpers/carve-guards.ts (ship maxSizeRatio re-measured: 309,483 / 187,706 = 1.6488).

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

docs/pregate.md

@@ -340,7 +340,7 @@ held-out and transcript layout.
 
 ```
 gstack-issue-links [--base <ref>] [--issue <n>]... [--ci-run <id>]... [--supersedes <pr>]... [--credit <pr>]...
-                   [--no-auto-links] [--repo <owner/repo>] [--out issues.json] [--json]
+                   [--no-auto-links] [--author <login>] [--repo <owner/repo>] [--out issues.json] [--json]
 ```
 
 Reads `origin/<base>..HEAD`. `Fixes #N` only on explicit closure intent: a
@@ -361,7 +361,8 @@ that names that run (its `flake:json` `first_seen.run_id`, or this repository's
 `Flake: <test>` title). Two issues for the same test keep the lower number;
 every refused match is printed with its reason.
 
-Open PRs by other authors (not the `gh` user, not this branch) whose closing
+Open PRs by other authors (not `--author`, default the `gh` viewer, which is
+an app identity on hosts like Capy, so pass your login there; 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`
 replaces that line and `Contributed by @author` is added (the credit lines of