fix(contracts): dedupe Environment types into a shared contract

[OnlyTerp]

Summary

npm run check:contracts currently fails on main:

Duplicate exported type/interface declarations found:
- Environment
- EnvironmentEngineId
  controller/src/modules/environments/types.ts
  frontend/src/lib/types.ts

Same playbook as #159: one declaration, re-exports everywhere else.

• New shared/contracts/environments.ts holds the single Environment + EnvironmentEngineId declarations (registered in validate-shared-contracts.mjs contract names + allowed files).
• controller/src/modules/environments/types.ts and frontend/src/lib/types.ts become re-exports.
• The controller-only EnvironmentId brand + asEnvironmentId are dropped — they were used in exactly one place (the serializer's return cast) and the wire shape is id: string on both sides anyway.

Test plan

• node scripts/validate-shared-contracts.mjs passes
• bun run typecheck (controller), tsc --noEmit + typecheck:desktop (frontend)
• environments-routes integration tests 4/4
• knip, jscpd, ui-structure, cycles, depcheck, production build (pre-push check:quality)

[OnlyTerp]

Closing — you beat me to it: 6532ce7 + 4fba584 landed the same consolidation into shared/contracts/environments (including dropping the id brand, 80fc4fa) while this was in flight. Race conditions of parallel agent loops.