refactor(cli): restructure cmd-*.ts into commands/ subdirectories

Reorganize flat cmd-*.ts files into commands/{workflow,thread,cas,init}/
subdirectories that strictly mirror the CLI subcommand hierarchy:

- workflow/: add, list, show, rm, history, rollback
- thread/: run, list, show, rm, fork, ps, kill, live, pause, resume
- cas/: get, put, list, rm, gc
- init/: workspace, template

Each group has an index.ts re-export. Split multi-command files
(cmd-cas.ts, cmd-thread.ts, cmd-init.ts) into per-subcommand files.
Rename cmd-help.ts → skill.ts to match the primary command name.

Update all import paths in cli-dispatch.ts and test files.

Pure structural change — no logic modifications.

Ref: #93, closes #94

packages/cli-workflow/__tests__/gc-cli.test.ts

@@ -10,7 +10,7 @@ import {
   getGlobalCasDir,
   putContentMerkleNode,
 } from "@uncaged/workflow";
-import { cmdThreadRemove } from "../src/cmd-thread.js";
+import { cmdThreadRemove } from "../src/commands/thread/rm.js";
 import { pathExists } from "../src/fs-utils.js";
 
 const cliEntryPath = fileURLToPath(new URL("../src/cli.ts", import.meta.url));