feat: add RPC server mode - #32

`codework serve` boots the harness once and serves session, sandbox, and event
RPCs over a WebSocket; `codework run --server <url>` drives a session remotely
through the same contract.

Harness changes this required:

- Execution lifecycle events: a busy period publishes `session.execution.started`
  and exactly one of `succeeded`, `failed`, or `interrupted`. A failure carries a
  namespaced category rather than a squashed cause string, and an interruption
  carries the reason its caller gave instead of one inferred from the cause.
- `Control.interrupt` acknowledges without waiting and returns whether there was
  work to stop. `awaitSettlement` waits for that execution alone, where
  `awaitIdle` follows successors admitted during its cleanup.
- Plugins declare the events they publish with `Plugin.define({ events })`.
  `EventRegistry.flatten` collects them before the event system builds and fails
  the boot on a kernel collision, a duplicate across plugins, or a type outside
  `plugin.<plugin-id>.*`.
- `optional()` used the source schema as its own `decodeTo` target, so nested
  transforms ran twice: an Option encoded to null, then null re-encoded as an
  Option.

Events reach clients through one feed that encodes each event once and fans it
out, so a slow client fails alone and a type nothing declared never costs a
client its queue capacity. Sessions are shared, so a terminal event arriving
before the client's own prompt is promoted is remembered rather than raised, and
the client reconciles against `session.message` before reporting — the volatile
stream is never the last word.