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/system-prompt/src/index.ts

@@ -73,7 +73,7 @@ export class SystemPrompt extends Service {
    * fiber is disposed. Emits `system-prompt/change` on register/unregister.
    */
   section(section: PromptSection): () => void {
-    return this.ctx.effect(() => {
+    const dispose = this.ctx.effect(() => {
       this.sections.push(section)
       this.ctx.emit('system-prompt/change')
       return () => {
@@ -82,6 +82,9 @@ export class SystemPrompt extends Service {
         this.ctx.emit('system-prompt/change')
       }
     }, 'systemPrompt.section()')
+    // ctx.effect's disposer returns Promise<void>; our disposer API is
+    // synchronous fire-and-forget — discard the (always-resolved) promise.
+    return () => void dispose()
   }
 
   /**
@@ -90,7 +93,7 @@ export class SystemPrompt extends Service {
    * removed when the calling fiber is disposed. Emits `system-prompt/change`.
    */
   tools(provider: () => ToolSchema[]): () => void {
-    return this.ctx.effect(() => {
+    const dispose = this.ctx.effect(() => {
       this.toolProviders.push(provider)
       this.ctx.emit('system-prompt/change')
       return () => {
@@ -99,6 +102,9 @@ export class SystemPrompt extends Service {
         this.ctx.emit('system-prompt/change')
       }
     }, 'systemPrompt.tools()')
+    // ctx.effect's disposer returns Promise<void>; our disposer API is
+    // synchronous fire-and-forget — discard the (always-resolved) promise.
+    return () => void dispose()
   }
 
   /**
@@ -113,7 +119,7 @@ export class SystemPrompt extends Service {
       sections: [...this.sections].sort((a, b) => a.order - b.order),
       tools: this.toolProviders.flatMap(provider => provider()),
     }
-    return this.ctx.waterfall(this, 'system-prompt/assemble', assembly, async () => assembly)
+    return this.ctx.waterfall(this, 'system-prompt/assemble', assembly, () => Promise.resolve(assembly))
   }
 }
 

packages/system-prompt/tests/system-prompt.spec.ts

@@ -8,7 +8,7 @@ describe('SystemPrompt', () => {
     await ctx.plugin(SystemPrompt)
 
     ctx.systemPrompt.section({ name: 'persona', order: 0, text: 'You are DeepSeek Code.' })
-    ctx.systemPrompt.section({ name: 'cwd', order: 20, text: () => `cwd: /tmp` })
+    ctx.systemPrompt.section({ name: 'cwd', order: 20, text: () => 'cwd: /tmp' })
     ctx.systemPrompt.section({ name: 'rules', order: 10, text: 'Be precise.' })
     ctx.systemPrompt.tools(() => [{ name: 'echo', description: 'echo back', parameters: {} }])