fix(host): bump Claude co-author trailer to Opus 4.8

The Claude host's commit co-author trailer was pinned to "Claude Opus
4.7". Opus 4.8 is the current model, so generated SKILL.md files
(document-generate, document-release, ship) attributed commits to a
stale model.

- hosts/claude.ts: canonical coAuthorTrailer 4.7 -> 4.8
- scripts/resolvers/utility.ts: fallback default 4.7 -> 4.8 (consistency)
- regenerated the 3 affected SKILL.md via `bun run gen:skill-docs`
- refreshed test/fixtures/golden/claude-ship-SKILL.md to match

Scope is intentionally limited to the co-author trailer. The eval-tier
model pinning (model-overlays/opus-4-7.md, claude-opus-4-7 eval fixtures)
is a separate, larger migration and is left untouched.

Fixes #1878

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

document-generate/SKILL.md

@@ -1145,7 +1145,7 @@ docs: generate [scope] documentation (Diataxis)
 
 Quadrants: [list which quadrants were produced]
 
-Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
+Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
 EOF
 )"
 ```

document-release/sections/release-body.md

@@ -196,7 +196,7 @@ committing.
 git commit -m "$(cat <<'EOF'
 docs: update project documentation for vX.Y.Z.W
 
-Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
+Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
 EOF
 )"
 ```

hosts/claude.ts

@@ -38,7 +38,7 @@ const claude: HostConfig = {
     linkingStrategy: 'real-dir-symlink',
   },
 
-  coAuthorTrailer: 'Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>',
+  coAuthorTrailer: 'Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>',
   learningsMode: 'full',
 };
 

scripts/resolvers/utility.ts

@@ -369,7 +369,7 @@ Minimum 0 per category.
 export function generateCoAuthorTrailer(ctx: TemplateContext): string {
   const { getHostConfig } = require('../../hosts/index');
   const hostConfig = getHostConfig(ctx.host);
-  return hostConfig.coAuthorTrailer || 'Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>';
+  return hostConfig.coAuthorTrailer || 'Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>';
 }
 
 export function generateChangelogWorkflow(_ctx: TemplateContext): string {

ship/SKILL.md

@@ -1174,7 +1174,7 @@ user via AskUserQuestion rather than destroying non-WIP commits.
 git commit -m "$(cat <<'EOF'
 chore: bump version and changelog (vX.Y.Z.W)
 
-Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
+Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
 EOF
 )"
 ```

test/fixtures/golden/claude-ship-SKILL.md

@@ -1174,7 +1174,7 @@ user via AskUserQuestion rather than destroying non-WIP commits.
 git commit -m "$(cat <<'EOF'
 chore: bump version and changelog (vX.Y.Z.W)
 
-Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
+Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
 EOF
 )"
 ```
@@ -1225,6 +1225,8 @@ git push -u origin <branch-name>
 
 ---
 
+**PR/MR title invariant (always applies — do not skip even if you don't open the section below):** Any PR or MR you create OR update in the next step MUST have a title that starts with `v$NEW_VERSION` (the version bumped in Step 12), in the format `v<NEW_VERSION> <type>: <summary>`. Never create or edit a PR/MR title without this prefix. Compute the correct title with the single source of truth helper: `~/.claude/skills/gstack/bin/gstack-pr-title-rewrite.sh "$NEW_VERSION" "<current title>"`. The full create/update procedure (idempotency, redaction scan, self-check) is in the section below.
+
 > **STOP.** Before syncing docs and creating or updating the PR/MR (Steps 18-19), Read `~/.claude/skills/gstack/ship/sections/pr-body.md` and execute it
 > in full. Do not work from memory — that section is the source of truth for this step.