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.

package.json

@@ -14,14 +14,19 @@
   "scripts": {
     "build": "tsc -b tsconfig.build.json && tsx scripts/build.ts",
     "typecheck": "tsc -b tsconfig.build.json && tsc -p tsconfig.typecheck.json",
+    "lint": "eslint .",
+    "lint:fix": "eslint . --fix",
     "test": "vitest run",
     "demo": "node --expose-internals --import tsx examples/echo-agent/start.ts"
   },
   "devDependencies": {
+    "@stylistic/eslint-plugin": "^5.10.0",
     "@types/node": "^25.3.5",
     "dumble": "^0.2.3",
+    "eslint": "^10.4.1",
     "tsx": "^4.22.4",
     "typescript": "^6.0.3",
+    "typescript-eslint": "^8.61.0",
     "vite-tsconfig-paths": "^6.1.1",
     "vitest": "^4.1.8"
   }