feat(cli): add channel-aware update notifier (#265)

* feat(cli): show cached update notices after commands

* docs(cli): describe update notices

* fix(cli): type update check environment

* fix(cli): decouple update notice display from refresh and harden suppression

Display a cached "update available" notice based solely on the lastNoticeAt
24h throttle, independent of checkedAt refresh freshness, matching the design's
independent display/refresh decisions. Suppress the check unconditionally under
--json, CI, and non-TTY before consulting output-mode preferences, so a
KTX_OUTPUT=pretty override can no longer make CI/non-TTY contexts phone npm.

packages/cli/src/update-check/channel.ts

@@ -0,0 +1,43 @@
+import semver from 'semver';
+
+export type UpdateChannel = 'latest' | 'next';
+
+export type UpdateDecision =
+  | { status: 'skip' }
+  | { status: 'upToDate'; channel: UpdateChannel; target: string }
+  | { status: 'available'; channel: UpdateChannel; target: string };
+
+/** @internal */
+export function inferUpdateChannel(installed: string): UpdateChannel | null {
+  const parsed = semver.parse(installed);
+  if (!parsed || installed === '0.0.0') {
+    return null;
+  }
+
+  const [prereleaseId] = parsed.prerelease;
+  if (prereleaseId === undefined) {
+    return 'latest';
+  }
+  if (prereleaseId === 'rc') {
+    return 'next';
+  }
+  return null;
+}
+
+export function decideUpdate(installed: string, distTags: Record<string, string>): UpdateDecision {
+  const channel = inferUpdateChannel(installed);
+  if (!channel || !semver.valid(installed)) {
+    return { status: 'skip' };
+  }
+
+  const target = distTags[channel];
+  if (!target || !semver.valid(target)) {
+    return { status: 'skip' };
+  }
+
+  if (semver.gt(target, installed)) {
+    return { status: 'available', channel, target };
+  }
+
+  return { status: 'upToDate', channel, target };
+}