chore(v→zig): delete legacy V-lang API stubs (#38)

Per estate-wide owner directive (2026-05-28): Zig is the default for
APIs/FFIs; Idris2 owns ABIs. Deletes the V-lang stub server and
protobuf/GraphQL descriptors at src/api/v/ + src/api/proto/opsm.pb.v +
src/api/graphql/schema.gql.v. The real adapter is src/opsm.zig +
src/abi/ffi.zig.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath