Preserve divergent branches during primary relocation

[andrei-hasna]

Summary

• Add explicit --preserve-divergent-branches-under for reviewed divergent branch preservation.
• Keep default relocation behavior fail-closed.
• Require exact pre-existing target refs for both preserved and target branch names; apply does not create Git refs.

Verification

• git diff --check origin/main...HEAD
• bun test src/db/primary-relocation.test.ts src/cli/primary-relocation.test.ts: 60 pass
• bun run typecheck
• bun test: 315 pass
• bun run build
• gitleaks git --log-opts=HEAD^..HEAD --redact --no-banner: no leaks

Reviews

• Data-safety review PASS, child goal 688c3371-ee03-4809-8a86-1c37fc780757.
• CLI/operator review PASS, child goal a83a5496-f2f6-4ce4-b95d-8ed796b46b9c.

No relocation apply, install, or registry mutation is included.