Fix relocation branch preservation review invariants

[andrei-hasna]

Summary

• Preserve branch identity as local vs remote throughout relocation review by blocking planned duplicate preserved local branch outputs.
• Add a regression where local and remote branch rows share a display name and would otherwise preserve to the same local key.
• Make PostgreSQL branch identity migrations reachable from remote sync schema preparation and export the migration runner.
• No relocation apply performed.

Verification

• bun test src/db/primary-relocation.test.ts — 71 pass
• bun test src/lib/auto-index.test.ts — 21 pass
• bun test src/lib/scanner.test.ts src/db/repos.test.ts — 52 pass
• bun test src/db/database.test.ts — 29 pass
• bun run typecheck — pass
• bun run build — pass
• git diff --check — pass
• gitleaks staged scan — no leaks
• gitleaks range scan 370dd0d..HEAD — no leaks across 8 commits

Independent review

• Reviewer A on 249d534: PASS, no P0/P1. Non-blocking P2/P3: PostgreSQL DDL/mock-only integration risk.
• Reviewer B on 249d534: PASS, no findings.

Notes

• External relocation apply remains separately gated and was not run.