feat: add /document-release skill with smart VERSION handling

New skill runs after /ship but before PR merge. Reads every doc file,
cross-references the diff, auto-updates factual changes, asks about
risky edits. CHANGELOG clobber protection: never uses Write tool on
CHANGELOG.md, only Edit with exact old_string matches.

Smart VERSION logic: instead of silently skipping already-bumped
versions, compares CHANGELOG entry scope against full diff and asks
if significant uncovered changes exist.

Also fixes gstack-upgrade/SKILL.md missing from skill-check.ts
SKILL_FILES array (existing inconsistency with gen-skill-docs.ts).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

test/skill-validation.test.ts

@@ -176,6 +176,7 @@ describe('Update check preamble', () => {
     'ship/SKILL.md', 'review/SKILL.md',
     'plan-ceo-review/SKILL.md', 'plan-eng-review/SKILL.md',
     'retro/SKILL.md',
+    'document-release/SKILL.md',
   ];
 
   for (const skill of skillsWithUpdateCheck) {
@@ -397,6 +398,7 @@ describe('No hardcoded branch names in SKILL templates', () => {
     'qa/SKILL.md.tmpl',
     'plan-ceo-review/SKILL.md.tmpl',
     'retro/SKILL.md.tmpl',
+    'document-release/SKILL.md.tmpl',
   ];
 
   // Patterns that indicate hardcoded 'main' in git commands
@@ -479,6 +481,7 @@ describe('v0.4.1 preamble features', () => {
     'ship/SKILL.md', 'review/SKILL.md',
     'plan-ceo-review/SKILL.md', 'plan-eng-review/SKILL.md',
     'retro/SKILL.md',
+    'document-release/SKILL.md',
   ];
 
   for (const skill of skillsWithPreamble) {