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.

examples/echo-agent/src/stdio-chat.ts

@@ -55,6 +55,6 @@ export function apply(ctx: Context) {
       setTimeout(() => process.exit(0), 200)
     })
     process.stdout.write('echo-agent ready. Type a message ("echo <text>" triggers the tool).\n> ')
-    return () => reader.close()
+    return () => { reader.close() }
   }, 'stdio-chat')
 }