oma session create persists env_secret to the secret store

POST /v1/sessions accepts env_secret (and inline github_repository.authorization_token) and persists each value through sessionSecrets.put, keyed by the minted resource id (identity-matched, not list-order zip).

• Fail closed with 500 when secret payloads are present but the store is unwired (no silent drop).
• Wired on Cloudflare route services and self-host Node (KvSessionSecretRepo over the existing SQL-backed KV).
• Session DELETE cascades deleteAllForSession (idempotent with the CF lifecycle path).
• API responses must not contain plaintext secret values.

Landed in #445 (closes audit #426). Restores documented env_secret behavior after a storage refactor dropped the create-path writer.

Why: Sandbox consumers (resource-mounter, github-creds, runtimes bundle) read the secret store; an empty store meant secrets never reached the sandbox despite 201 creates.

How to apply: Keep create-path sessionSecrets.put wired on both CF and Node. Prefer identity match over zip-by-index when attaching values to minted resources. Do not invent or log secrets.

Hub: project-open-managed-agents.