CODEOWNERS: stale domain paths after reorganisation

[sirdeggen]

Problem

CODEOWNERS still references paths that no longer exist and is missing the new middleware domain:

Entry	Status
/packages/helpers/	Does not exist — was dissolved
/apps/	Does not exist — apps moved to separate repos
/packages/middleware/	Missing — new domain containing go-bsv-middleware

Fix

# Remove:
/packages/helpers/              @bsv-blockchain/stack-architects
/apps/                          @bsv-blockchain/stack-architects

# Add:
/packages/middleware/           @bsv-blockchain/middleware-leads

Also consider adding /packages/network/ with a named owner — currently falls through to the catch-all.

Acceptance

• No CODEOWNERS entry points to a path that doesn't exist in the repo.
• packages/middleware/go-bsv-middleware changes require review from an appropriate team.