Add ESLint: typescript-eslint strict-type-checked + stylistic formatting

Flat config with two layers. Correctness (type-checked): the headline
rules for this codebase are no-floating-promises / no-misused-promises
(a lost promise in the agent loop is our primary bug class),
switch-exhaustiveness-check (we switch over merge-extensible unions
everywhere), no-unnecessary-condition, require-await, and
no-explicit-any. Style (@stylistic): 2-space, no semicolons, single
quotes, trailing commas, max-len 140 — the existing house style, now
enforced instead of drifting between agents. vendor/ is excluded
(vendored source keeps upstream style); tests relax the rules that
fight test ergonomics (non-null assertions after expects, async mock
signatures, non-Error throws).

Code adjusted to pass: registry disposers wrap ctx.effect's
promise-returning disposer behind a sync () => void (our public API),
BlockAssembler gains an invariant-checking mustGet instead of non-null
assertions, lastTurnNumber uses findLast, waterfall tails return
Promise.resolve instead of async-without-await arrows, and the two
deliberate suppressions (non-exhaustive derivation switch, unbound
execute pass-through) carry justification comments.

yarn lint / yarn lint:fix added.

packages/agent/src/index.ts

@@ -35,7 +35,7 @@ export class AgentRegistry extends Service {
    * when the calling fiber is disposed. Returns the disposer.
    */
   register(agent: Agent): () => void {
-    return this.ctx.effect(() => {
+    const dispose = this.ctx.effect(() => {
       if (this.store.has(agent.id)) {
         throw new Error(`agent "${agent.id}" is already registered`)
       }
@@ -46,6 +46,9 @@ export class AgentRegistry extends Service {
         this.ctx.emit('agent/disposed', agent)
       }
     }, 'agents.register()')
+    // ctx.effect's disposer returns Promise<void>; our disposer API is
+    // synchronous fire-and-forget — discard the (always-resolved) promise.
+    return () => void dispose()
   }
 
   get(id: string): Agent | undefined {