refactor: remove AI slop from refactored files

Behavior-preserving cleanup of AI-generated code smells in 5 files authored/moved by this PR:

- src/hooks/model-fallback/fallback-state-controller.ts (-47/+47 net reorganization, redundant defensiveness removed)
- src/shared/model-string-parser.ts (-4 LOC obvious-comment cleanup)
- src/shared/ripgrep-cli.ts (-13 LOC obvious comments + redundant defensive checks)
- src/tools/delegate-task/tool-description.ts (-6 LOC)
- src/tools/look-at/look-at-input-preparer.ts (-6 LOC)

Targets: obvious comments that restate code, over-defensive null checks on guaranteed values, redundant existence checks. No public API signatures changed, no type hints removed, no new abstractions introduced. Full test suite still passes.

src/shared/model-string-parser.ts

@@ -41,13 +41,13 @@ export function parseModelString(
   const trimmedModel = model.trim()
   if (!trimmedModel) return undefined
 
-  const parts = trimmedModel.split("/")
-  if (parts.length < 2) {
+  const separatorIndex = trimmedModel.indexOf("/")
+  if (separatorIndex === -1) {
     return undefined
   }
 
-  const providerID = parts[0]?.trim()
-  const rawModelID = parts.slice(1).join("/").trim()
+  const providerID = trimmedModel.slice(0, separatorIndex).trim()
+  const rawModelID = trimmedModel.slice(separatorIndex + 1).trim()
   if (!providerID || !rawModelID) {
     return undefined
   }