refactor: wave 1 - extract leaf modules, rename catch-all files, split index.ts hooks

- Split 25+ index.ts files into hook.ts + extracted modules
- Rename all catch-all utils.ts/helpers.ts to domain-specific names
- Split src/tools/lsp/ into ~15 focused modules
- Split src/tools/delegate-task/ into ~18 focused modules
- Separate shared types from implementation
- 155 files changed, 60+ new files created
- All typecheck clean, 61 tests pass

src/agents/builtin-agents/model-resolution.ts

@@ -0,0 +1,28 @@
+import { resolveModelPipeline } from "../../shared"
+
+export function applyModelResolution(input: {
+  uiSelectedModel?: string
+  userModel?: string
+  requirement?: { fallbackChain?: { providers: string[]; model: string; variant?: string }[] }
+  availableModels: Set<string>
+  systemDefaultModel?: string
+}) {
+  const { uiSelectedModel, userModel, requirement, availableModels, systemDefaultModel } = input
+  return resolveModelPipeline({
+    intent: { uiSelectedModel, userModel },
+    constraints: { availableModels },
+    policy: { fallbackChain: requirement?.fallbackChain, systemDefaultModel },
+  })
+}
+
+export function getFirstFallbackModel(requirement?: {
+  fallbackChain?: { providers: string[]; model: string; variant?: string }[]
+}) {
+  const entry = requirement?.fallbackChain?.[0]
+  if (!entry || entry.providers.length === 0) return undefined
+  return {
+    model: `${entry.providers[0]}/${entry.model}`,
+    provenance: "provider-fallback" as const,
+    variant: entry.variant,
+  }
+}