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>

.github/workflows/automerge.yml

@@ -8,12 +8,11 @@ jobs:
       - uses: actions/checkout@v4
       - uses: oven-sh/setup-bun@v2
         with:
-          bun-version: latest
+          bun-version: '1.3.1'
       - name: Install dependencies
         run: bun install
       - name: Lint
         run: bun run lint
-        continue-on-error: true
       - name: Build
         run: bun run build
   automerge: