fix(coord): avoid onboarding slug collisions across repository owners

Follow-up from PR #375 review.

## Problem
`apps/coord-gateway/src/onboard.ts` derives a global slug from the repository short name only. `owner-a/demo` and `owner-b/demo` can collide.

## Expected behavior
Global identity includes owner and repository, or uses another collision-resistant canonical mapping.

## Verification
Onboard two repositories with identical short names under different owners and assert distinct, stable workspace identities.

Start in apps/coord-gateway/src/onboard.ts, where the global slug is derived from the repository short name. Trace the onboarding identity mapping and review PR #375 for context. Done means onboarding owner-a/demo and owner-b/demo produces distinct, stable workspace identities, with verification covering both repositories.

Written by the indexing model from the issue text.