[VDR][v0.6.0] Manifest contract ships incomplete

[anwithk]

Theme issue grouping v0.6.0-cycle VDR findings: Manifest contract ships incomplete.

Finding	Severity	Summary
API-32d13388	🟠 high	New EnvironmentManifest contract ships with zero adopters repo-wide
DOC-d5e74f19	🟠 high	manifest.yaml is the new required onboarding contract with no schema reference or example

Child issues are linked as sub-issues below.

_{DX review Gym-1-20260821203248 — incremental from v0.5.0 to fa0d25158. Parent tracker #2214.}