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/tools/tests/tools.spec.ts

@@ -18,7 +18,7 @@ const echoTool = defineTool({
   description: 'echo arguments back',
   parameters: { text: { type: 'string' } },
   async execute(args) {
-    return [{ type: 'text' as const, text: String(args.text ?? '') }]
+    return [{ type: 'text' as const, text: args.text ?? '' }]
   },
 })
 
@@ -372,7 +372,7 @@ describe('schema DSL regressions (Codex review round 2)', () => {
       ...echoTool,
       name: 'object-thrower',
       async execute() {
-        // eslint-disable-next-line no-throw-literal — testing non-Error throws
+        // testing non-Error throws on purpose
         throw { message: 'denied by object' }
       },
     })