Runtime portability layer (serverless → portable Node core)

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Part of #975.

Context

The automation core currently depends on a serverless platform's runtime — its request handler, scheduled/cron triggers, queue consumer, and environment bindings. To self-host, the same core must run on a plain Node runtime without any platform-specific globals.

Deliverables

• A runtime/platform interface abstracting: HTTP webhook intake, scheduled execution, the work-queue consumer, and the environment/bindings accessor.
• The shared automation core decoupled from platform-specific globals so it compiles and runs on both the hosted platform and Node (no platform imports leak into the shared core).
• A Node entrypoint that implements the interface (HTTP server + scheduler + queue worker).
• Build targets for both platforms; CI builds both.

Acceptance criteria

• The core produces identical behavior on the hosted platform and the Node runtime.
• A lint/build guard prevents platform-specific imports from entering the shared core.

Out of scope

• Review heuristics / decision policy (unchanged; ported as-is, internals stay server-side).

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Closing as completed (2026-07-01 issue audit): Portable Node core shipped (src/server.ts, src/selfhost/cf-workers-shim.ts, selfhost queue consumers).