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/src/commands/thread/rm.ts

@@ -0,0 +1,28 @@
+import { unlink } from "node:fs/promises";
+import { dirname, join } from "node:path";
+
+import { err, garbageCollectCas, ok, type Result } from "@uncaged/workflow";
+
+import { resolveThreadDataPath } from "../../thread-scan.js";
+
+export async function cmdThreadRemove(
+  storageRoot: string,
+  threadId: string,
+): Promise<Result<void, string>> {
+  const dataPath = await resolveThreadDataPath(storageRoot, threadId);
+  if (dataPath === null) {
+    return err(`thread not found: ${threadId}`);
+  }
+
+  const dir = dirname(dataPath);
+  const infoPath = join(dir, `${threadId}.info.jsonl`);
+  const runningPath = join(dir, `${threadId}.running`);
+
+  await unlink(dataPath);
+  await unlink(infoPath).catch(() => {});
+  await unlink(runningPath).catch(() => {});
+
+  await garbageCollectCas(storageRoot);
+
+  return ok(undefined);
+}