[Feat] Handle @roomote mentions on GitLab issues

[roomote-roomote]

Opened on behalf of Matt Rubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

Related issue

Internal P1: GitLab @roomote mentions on issues were a silent no-op.

Why this PR exists

• A maintainer explicitly invited this PR in the linked issue or discussion
• I am a maintainer / this is internal Roomote work

What changed

GitLab Note webhooks now handle @roomote mentions on Issues, not only Merge Requests.

• First mention starts a standard task linked to the GitLab issue and posts an acknowledgement note on the issue
• A later mention on the same issue reuses/resumes that task instead of spawning a sibling
• Issue reuse lookup is host-scoped (target.repo.host → payload sourceControlHost, with legacy-null tolerance) so same-namespace issues on different GitLab instances cannot cross-match
• System notes and Roomote/bot-authored notes still stay filtered
• Existing Merge Request mention → review-task flow is unchanged
• Adds createGitLabIssueNote

How it was tested

• pnpm exec dotenvx run -f .env.test -- pnpm --filter @roomote/api exec vitest run src/handlers/gitlab/__tests__/handleNote.test.ts (24 passed)
• pnpm exec dotenvx run -f .env.test -- pnpm --filter @roomote/db exec vitest run src/lib/__tests__/github-branch-activity.test.ts -t "findReusableGitHubIssueTaskOwner" (6 passed)
• Pre-push gates (oxlint, residual lint, check-types:fast, knip) passed on push

Checklist

• The PR title follows the repo convention: [Fix], [Feat], [Improve], [Refactor], [Docs], or [Chore] followed by a user-facing description
• This PR is small and scoped to one change
• pnpm lint and pnpm check-types pass locally
• I added tests or included a clear manual validation note above
• I removed secrets, tokens, private keys, and customer data from code, logs, and screenshots
• If this change should appear in the changelog, I ran pnpm changeset

[roomote-roomote]

No new code issues found. See task

• apps/api/src/handlers/gitlab/handleNote.ts:427 - Reuse lookup does not receive or apply target.repo.host; GitLab issues with the same namespace and IID on different instances can route follow-up work to the other instance's task. Thread the host into findReusableGitHubIssueTaskOwner and filter sourceControlHost with the existing legacy-null fallback.

_{Reviewed d8e9992}

[roomote-roomote]

Fixed host-scoping on GitLab issue task reuse.

findReusableGitHubIssueTaskOwner now accepts host and matches payload sourceControlHost (or legacy null). The GitLab issue mention path threads target.repo.host into both the reuse lookup and the launched payload.

Pushed in d8e9992