feat(onboarding): automated re-onboarding on default-branch drift

**Context:** ROADMAP.md → Automated re-onboarding

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## Component

API or orchestration

## Describe the feature

**Event-driven refresh** of blueprint-related artifacts when the default branch changes materially (GitHub webhook); optional EventBridge schedule for periodic drift checks. Distinct from **scheduled triggers** (task creation).

## Use case

Repo layout, dependencies, and conventions drift; stale blueprint context misleads agents. Operators should not manually re-onboard after every structural change.

## Proposed solution

1. Webhook: `push` to default branch → evaluate materiality (path filters, diff size).
2. Refresh: re-index memory hints, regenerate dynamic artifacts (#388), update Blueprint metadata.
3. Optional weekly drift job for quiet repos.
4. Notify operator on significant refresh (Slack/email).
5. Document triggers in `docs/design/REPO_ONBOARDING.md`.

## Other information

- Distinct from #389 (cron task creation).
- Design context: `docs/design/REPO_ONBOARDING.md`.

- [ ] This might be a breaking change

Start with docs/design/REPO_ONBOARDING.md and trace the existing API or orchestration entry points for onboarding, dynamic artifact generation (#388), and task triggers (#389). Define how a default-branch GitHub push is judged material, how refresh and operator notification are handled, and whether the optional EventBridge schedule belongs in scope. Done includes documented trigger behavior and an agreed implementation design.

Written by the indexing model from the issue text.