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/tests/mock-adapter.ts

@@ -5,7 +5,7 @@ import { LlmAdapter } from '@deepseek-ai/dsh-llm'
 export function textResponse(text: string): StreamChunk[] {
   return [
     { type: 'block-start', index: 0, blockType: 'text' },
-    ...[...text].map((char): StreamChunk => ({ type: 'text-delta', index: 0, text: char })),
+    ...Array.from(text, (char): StreamChunk => ({ type: 'text-delta', index: 0, text: char })),
     { type: 'block-end', index: 0, block: { type: 'text', text } },
     { type: 'usage', usage: { inputTokens: 10, outputTokens: text.length } },
     { type: 'finish', reason: { kind: 'stop' } },
@@ -60,8 +60,8 @@ export class MockAdapter extends LlmAdapter {
       yield { type: 'block-start', index: 0, blockType: 'text' }
       yield { type: 'text-delta', index: 0, text: 'partial' }
       await new Promise<void>((_resolve, reject) => {
-        if (options.signal?.aborted) return reject(new Error('aborted'))
-        options.signal?.addEventListener('abort', () => reject(new Error('aborted')), { once: true })
+        if (options.signal?.aborted) { reject(new Error('aborted')); return }
+        options.signal?.addEventListener('abort', () => { reject(new Error('aborted')) }, { once: true })
       })
       return
     }