Replace mutable Fastly runtime selectors with version-scoped descriptors

[aram356]

Problem

Fastly has no process environment at runtime. PR #344 compensated with mutable selector entries, service-scoped selector names, and staging selector stores. That model makes the canonical EDGEZERO__STORES__... variables ineffective at deployment boundaries and creates collision, isolation, and publication-order problems across services and versions.

Deployment environments should select runtime values and physical resources without changing application release artifacts or embedding a Fastly service ID in environment-variable names.

Expected behavior

• One application release has byte-identical CLI, package, edgezero.toml, and fastly.toml artifacts across publishers, production, and staging.
• The selected deployment environment supplies canonical EDGEZERO__... runtime values. Environment variables do not include a Fastly service ID.
• Production and staging may select the same or different Config, KV, and Secret Stores.
• Secret Stores remain optional when the application manifest does not declare one.
• The generic CLI passes typed, adapter-neutral deployment context. The Fastly adapter owns Fastly resource linking and publication.
• Fastly uses one physical edgezero_runtime_env Config Store. Each service version reads an immutable descriptor at EDGEZERO__SERVICES__<SERVICE_ID>__VERSIONS__<VERSION>__ENV_V1.
• The descriptor and exact physical resource links are written and verified before the version is staged or activated.
• Config push, config diff, and deploy resolve the same Config Store key from the selected environment, applicable manifest defaults, and the target fallback.

Acceptance criteria

• Remove the service-scoped selector inputs, mutable selector reconciliation, staging selector twins, and EDGEZERO_FASTLY_BUILD_SETTINGS introduced by Support optional typed secret paths and Fastly store mappings #344.
• Do not migrate or fall back to Support optional typed secret paths and Fastly store mappings #344 selector entries; old entries and staging twins are inert after cutover.
• Keep one physical edgezero_runtime_env store and scope immutable runtime descriptors by Fastly service and version.
• Reconcile declared Config, KV, and optional Secret Store links on the draft version before publication, including removal of inherited production links that are not selected for the target.
• Fail before staging or activation when descriptor writes, readback verification, resource-link reconciliation, or final source checks fail.
• Preserve unregistered custom adapter commands that do not require the Fastly-managed lifecycle.
• Keep lifecycle actions provider-specific while the CLI and application release contract remain adapter-independent.
• Remove managed-action inputs that can select a runtime key independently from deploy.
• Document the clean-cutover model, environment precedence, production/staging behavior, recovery, and serialization expectations.
• Cover the runtime descriptor, exact link reconciliation, publication ordering, config resolution, optional secrets, actions, and documentation with tests.

Deploys for a Fastly service must be serialized. Concurrent operator deletion or mutation remains an external provider error; the deployment path should fail clearly without adding recovery machinery for every possible external race.