feat(webhook): assigned/milestoned/edited branches + write-lock echo skip

Expand the GitHub issues webhook handler with new action branches:
- assigned/unassigned: full-replace report_assignees with write-lock echo check
- milestoned/demilestoned: update milestone columns with write-lock echo check
- edited (title): update reports.title with write-lock echo skip

Add companion integration tests (6 passing) covering each new branch and
both echo-skip scenarios. Fix the dev-server webhook secret by setting
GITHUB_APP_* defaults in apps/dashboard/.env so the server can verify HMAC
signatures during local test runs.

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

Author: Ripwords