fix: use FORGE_API_BASE for /repos/migrate endpoint, build payload with jq

- FORGE_API is repo-scoped; /repos/migrate needs the global FORGE_API_BASE
- Use jq -n --arg for safe JSON construction (no shell interpolation)
- Update docs to reference FORGE_API_BASE

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/mirror-bootstrap.md

@@ -8,7 +8,7 @@ How to populate an empty Forgejo repo from an external source using
 | Variable | Example | Purpose |
 |---|---|---|
 | `FORGE_URL` | `http://forgejo:3000` | Forgejo instance base URL |
-| `FORGE_API` | `${FORGE_URL}/api/v1` | API base (set by `lib/env.sh`) |
+| `FORGE_API_BASE` | `${FORGE_URL}/api/v1` | Global API base (set by `lib/env.sh`) |
 | `FORGE_TOKEN` | (admin or org-owner token) | Must have `repo:create` scope |
 
 The target org/user must already exist on the Forgejo instance.