fix(cliproxy): respect version pin when user installs specific version

When a user explicitly installs a specific CLIProxy version via
`ccs cliproxy --install <version>`, this version should persist and
not be auto-updated. Previously, subsequent commands would check for
updates and overwrite the user's choice.

Changes:
- Add .version-pin file to persist user's explicit version choice
- Update ensureCLIProxyBinary() to check for version pin and skip auto-update
- Update --install command to save version pin with "(pinned)" confirmation
- Update --latest command to clear version pin and enable auto-update
- Add --update command as alias for --latest (for unpinning workflow)
- Update status display to show "(pinned)" indicator when version is pinned
- Update help text to document new pin/unpin behavior

Closes #88

src/cliproxy/index.ts

@@ -43,6 +43,11 @@ export {
   getInstalledCliproxyVersion,
   installCliproxyVersion,
   fetchLatestCliproxyVersion,
+  getPinnedVersion,
+  savePinnedVersion,
+  clearPinnedVersion,
+  isVersionPinned,
+  getVersionPinPath,
 } from './binary-manager';
 
 // Config generation