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/session/src/index.ts

@@ -97,6 +97,10 @@ export class Session {
   deriveMessages(): Message[] {
     const messages: Message[] = []
     for (const event of this.log) {
+      // Intentionally non-exhaustive: only message-producing events derive
+      // history; turn/step boundaries, chunks, usage, and errors are
+      // trace/replay data.
+      // eslint-disable-next-line @typescript-eslint/switch-exhaustiveness-check
       switch (event.type) {
         case 'user/message': {
           messages.push({ role: 'user', content: event.data.content })
@@ -155,7 +159,7 @@ export class SessionStore extends Service {
     if (this.store.has(id)) throw new Error(`session "${id}" already exists`)
     const session = new Session(id, seed)
     this.ctx.effect(() => {
-      session.onAppend = (event) => this.ctx.emit('session/event', session, event)
+      session.onAppend = (event) => { this.ctx.emit('session/event', session, event) }
       this.store.set(id, session)
       this.ctx.emit('session/created', session)
       return () => {