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.

eslint.config.mjs

@@ -0,0 +1,133 @@
+import stylistic from '@stylistic/eslint-plugin'
+import tseslint from 'typescript-eslint'
+
+/**
+ * ESLint flat config. Two layers:
+ *
+ * 1. typescript-eslint strict-type-checked — correctness rules that need the
+ *    type checker. The headline rules for this codebase: no-floating-promises
+ *    and no-misused-promises (an un-awaited promise in the agent loop is our
+ *    primary bug class), switch-exhaustiveness-check (we switch over
+ *    merge-extensible unions everywhere).
+ * 2. @stylistic — formatting (2-space, no semicolons, single quotes, trailing
+ *    commas), so style is enforced rather than drifting between agents.
+ *
+ * vendor/ is linted lightly (style only stays OFF — vendored code keeps
+ * upstream style; only a few safety rules apply there) and examples/tests are
+ * linted with relaxed unsafe-* rules where mocks intentionally bend types.
+ */
+export default tseslint.config(
+  {
+    ignores: [
+      '**/lib/**',
+      '**/node_modules/**',
+      'vendor/**', // vendored source keeps upstream style and idioms
+      '**/*.js',
+      '**/*.mjs',
+    ],
+  },
+
+  // --- our packages: full strictness -------------------------------------
+  {
+    files: ['packages/*/src/**/*.ts', 'examples/**/*.ts', 'scripts/**/*.ts'],
+    extends: [
+      ...tseslint.configs.strictTypeChecked,
+    ],
+    languageOptions: {
+      parserOptions: {
+        project: ['./tsconfig.typecheck.json'],
+        tsconfigRootDir: import.meta.dirname,
+      },
+    },
+    rules: {
+      // The bug class this repo cares most about: lost promises in the loop.
+      '@typescript-eslint/no-floating-promises': 'error',
+      '@typescript-eslint/no-misused-promises': 'error',
+      '@typescript-eslint/require-await': 'error',
+      '@typescript-eslint/switch-exhaustiveness-check': ['error', {
+        considerDefaultExhaustiveForUnions: true,
+      }],
+      '@typescript-eslint/no-unnecessary-condition': ['error', {
+        allowConstantLoopConditions: true,
+      }],
+      // `any` requires a justification comment — enforced as: no bare casts.
+      '@typescript-eslint/no-explicit-any': 'error',
+      // Style points where the codebase intentionally diverges from preset:
+      '@typescript-eslint/no-namespace': 'off', // Cordis Config-namespace idiom
+      '@typescript-eslint/no-empty-object-type': 'off', // merge-extensible maps
+      '@typescript-eslint/no-invalid-void-type': 'off', // event signatures
+      '@typescript-eslint/restrict-template-expressions': ['error', {
+        allowNumber: true,
+        allowBoolean: true,
+      }],
+      // `void foo()` in arrow listeners is our idiom for intentional fire-and-forget
+      'no-void': 'off',
+      '@typescript-eslint/no-unused-vars': ['error', {
+        argsIgnorePattern: '^_',
+        varsIgnorePattern: '^_',
+        caughtErrorsIgnorePattern: '^_',
+      }],
+    },
+  },
+
+  // --- examples: demo code conforms to async interfaces without awaiting ---
+  {
+    files: ['examples/**/*.ts'],
+    rules: {
+      '@typescript-eslint/require-await': 'off',
+    },
+  },
+
+  // --- tests: same rules, minus the friction that fights test ergonomics --
+  {
+    files: ['packages/*/tests/**/*.ts'],
+    extends: [
+      ...tseslint.configs.strictTypeChecked,
+    ],
+    languageOptions: {
+      parserOptions: {
+        project: ['./tsconfig.typecheck.json'],
+        tsconfigRootDir: import.meta.dirname,
+      },
+    },
+    rules: {
+      '@typescript-eslint/no-floating-promises': 'error',
+      '@typescript-eslint/no-misused-promises': 'error',
+      '@typescript-eslint/no-explicit-any': 'error',
+      '@typescript-eslint/no-non-null-assertion': 'off', // assertions follow expect()s
+      '@typescript-eslint/no-unnecessary-condition': 'off',
+      '@typescript-eslint/require-await': 'off', // mock execute() signatures
+      '@typescript-eslint/no-empty-function': 'off', // stub agents
+      '@typescript-eslint/only-throw-error': 'off', // testing non-Error throws
+      '@typescript-eslint/no-namespace': 'off',
+      '@typescript-eslint/no-empty-object-type': 'off',
+      '@typescript-eslint/restrict-template-expressions': 'off',
+      '@typescript-eslint/no-unused-vars': ['error', {
+        argsIgnorePattern: '^_',
+        varsIgnorePattern: '^_',
+        caughtErrorsIgnorePattern: '^_',
+      }],
+    },
+  },
+
+  // --- formatting (everything we own) -------------------------------------
+  {
+    files: ['packages/**/*.ts', 'examples/**/*.ts', 'scripts/**/*.ts', 'eslint.config.mjs'],
+    plugins: { '@stylistic': stylistic },
+    rules: {
+      '@stylistic/indent': ['error', 2],
+      '@stylistic/semi': ['error', 'never'],
+      '@stylistic/quotes': ['error', 'single', { avoidEscape: true }],
+      '@stylistic/comma-dangle': ['error', 'always-multiline'],
+      '@stylistic/eol-last': ['error', 'always'],
+      '@stylistic/no-trailing-spaces': 'error',
+      '@stylistic/object-curly-spacing': ['error', 'always'],
+      '@stylistic/arrow-parens': ['error', 'as-needed', { requireForBlockBody: true }],
+      '@stylistic/member-delimiter-style': ['error', {
+        multiline: { delimiter: 'none' },
+        singleline: { delimiter: 'semi', requireLast: false },
+      }],
+      '@stylistic/max-len': ['error', { code: 140, ignoreUrls: true, ignoreStrings: true, ignoreTemplateLiterals: true }],
+    },
+  },
+)