refactor(cli): Phase 4 cleanup — dedup, extract, deprecate

- bundle-store.ts: remove private pathExists, import from fs-utils
- thread-scan.ts: extract parseFirstJsonLineObject helper, dedup first-line parsing
- commands/workflow/add-argv.ts: extract parseAddArgv from add.ts
- commands/thread/fork-argv.ts: extract parseForkArgv from fork.ts
- commands/cas/*.ts: remove unused _threadId params from cmdCas* functions
- cli-dispatch.ts: add deprecation warning to help command
- commands/init/templates.ts: extract template strings from template.ts
- cli-color.ts: extract shouldUseColor, highlightLiveRole, dimGreyLine from live.ts
- 242 tests pass, biome clean

Closes #97

packages/cli-workflow/src/bundle-store.ts

@@ -1,16 +1,9 @@
-import { mkdir, readFile, stat, writeFile } from "node:fs/promises";
+import { mkdir, readFile, writeFile } from "node:fs/promises";
 import { join } from "node:path";
 
 import { err, ok, type Result } from "@uncaged/workflow";
 
-async function pathExists(path: string): Promise<boolean> {
-  try {
-    await stat(path);
-    return true;
-  } catch {
-    return false;
-  }
-}
+import { pathExists } from "./fs-utils.js";
 
 export type BundleFileSource = { kind: "text"; text: string } | { kind: "path"; path: string };