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-loop/src/agent.ts

@@ -65,7 +65,7 @@ export class LoopAgent implements Agent {
 
   steer(content: ContentBlock[], options?: SendOptions): void {
     if (this._status === 'disposed') throw new Error(`agent "${this.id}" is disposed`)
-    if (this._status !== 'running') return this.send(content, options)
+    if (this._status !== 'running') { this.send(content, options); return }
     const source = this.resolveSource(options)
     this.inbox.steer({ content, source })
     this.ctx.emit('agent/queued', this, content, { source, steering: true })
@@ -88,7 +88,7 @@ export class LoopAgent implements Agent {
    */
   start(): () => void {
     this.done = runLoop(this.ctx, this, {
-      setStatus: status => this.setStatus(status),
+      setStatus: (status) => { this.setStatus(status) },
       setAbort: controller => void (this.currentAbort = controller),
       disposed: this.disposed,
       isDisposed: () => this._status === 'disposed',