feat(proto): restructure identity protos into three bounded services (#9)

Replace monolithic proto/authorization/ and proto/identity/ with three
bounded service contracts:

- identity.v1.IdentityService: ResolveIdentity, Register, ListProviders
- session.v1.SessionService: CreateSession, ValidateSession, RevokeSession,
  RevokeUserSessions, ListUserSessions, SubscribeSessionEvents (stream)
- directory.v1.DirectoryService: GetUser, GetUserByEmail, GetTenant,
  CreateTenant, UpdateTenant

These are the contracts janus implements and other mesh services consume.

Closes #7

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: case-zbz