[Improve] Name the source-control platform in PR review notifications

[roomote-roomote]

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

What changed

Self-review follow-up messages now name the source-control platform, so people see wording like “I reviewed … on GitHub” (or GitLab, Gitea, Azure DevOps) instead of only a linked repo/PR reference.

The review-notification triage prompt now receives the provider label and is instructed to include that platform on first-person self-review messages. GitHub review-activity and review-summary enqueues also stamp sourceControlProvider: 'github' so the label is available to triage.

Why this change was made

Those messages could look opaque in chat: a link like owner/repo#123 does not make it obvious which host the review ran on. Spelling the platform in the sentence makes the conversation clearer at a glance.

Impact

After this ships, automated self-review notifications should read more clearly in Slack and other conversation surfaces, for example:

I reviewed RooCodeInc/Roomote#138 on GitHub and found no code issues. …

Third-party reviewer notifications are unchanged and still do not force a platform name. Non-GitHub providers already have provider plumbing if those notification paths are wired later.

[roomote-roomote]

No code issues found. See task

Reviewed the provider-label plumbing end to end: GitHub webhook handlers stamp sourceControlProvider: 'github', the enqueue path forwards it onto the queued request, the worker passes it through preparePrReviewNotificationDelivery, and triagePrReviewActivity normalizes it into the prompt label plus the updated self-review system-prompt instructions. Provider fallback stays github, so existing behavior is unchanged. Both updated test suites pass locally.