fix(workflows): fix IS_PRERELEASE bug, hotfix changelog, dependabot target, back-merge automation, lint gate, and manifest version mismatch

- fix version-bump.yml: IS_PRERELEASE was never defined, breaking iterative beta versioning (7.x.x-beta.1 → 7.x.x-beta.2)
- fix hotfix-release.yml: move changelog generation before merge so git log captures commits correctly
- fix dependabot.yml: was empty; now targets dev branch to keep Dependabot PRs in the normal dev→beta→main flow
- fix automerge.yml: remove continue-on-error from lint step so lint failures block Dependabot auto-merge; pin bun-version to 1.3.1
- fix production-release.yml: add automated back-merge of main→beta and main→dev after each production release
- fix manifest/chrome.json and manifest/firefox.json: version was 7.6.0, mismatched package.json 7.6.1

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

manifest/chrome.json

@@ -4,7 +4,7 @@
   "default_locale": "en",
   "name": "__MSG_name__",
   "description": "__MSG_description__",
-  "version": "7.6.0",
+  "version": "7.6.1",
   "homepage_url": "https://muetab.com",
   "permissions": ["search", "bookmarks"],
   "action": {

manifest/firefox.json

@@ -2,7 +2,7 @@
   "manifest_version": 3,
   "name": "Mue",
   "description": "Fast, open and free-to-use new tab page for modern browsers.",
-  "version": "7.6.0",
+  "version": "7.6.1",
   "homepage_url": "https://muetab.com",
   "permissions": ["bookmarks"],
   "action": {