Require the installation ID and credential key

[SaladDay]

Core now always runs with an installation ID and a credential key. The run modes without them are deleted.

• Process settings. processconfig.Load requires both files, and OAC_PUBLIC_URL with them. A missing file is a typed configuration error that oac check-config reports. main always builds the sandbox manager.
• Deleted modes. The nil key and nil ID branches, "credential storage is disabled", credential_storage_unavailable, and the "not configured" 503s. Constructors take a required key or ID. Tests share pgtest.CredentialKey.
• Admin API. installation_id in GET /core/v1/installation is a required string (make openapi; admin-api.md en and zh). Web's null branches are gone.
• Producers. The dev run and the acceptance harnesses write both files and mount them. The installation ID stays stable for a given database, because Core claims it there: official_client.py uses a fixed test UUID, and the README tells developers to keep the ID and key with their database.
• Lost key. Under a new key, Core starts, credentials list, deletion works and nothing leaks. A credential sealed with the old key returns the existing typed error. If an old-key E2B credential is stored, hosted execution maps it to 503 execution_unavailable. The recovery path in vaults.md (en and zh): Reset deployment and set up again. Leftover E2B sandboxes expire under their E2B timeout.
• Docs. configuration.md, vaults.md, admin-api.md, sessions-events, runtime-observability and IMPLEMENTATION.md, en and zh where they exist.

Net lines: non-test −283, test −303.

Checks: go build, go vet, make openapi (no diff); Go tests for every changed and dependent package against a database, including cmd/server, api, execution, processconfig, sandbox/providers and tests/integration; Web and agents-client typecheck and the touched vitest files; docs build, translations and make check-docs. The acceptance scripts changed, so this PR waits for CI before merging.

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