feat: add self-update command, update README with install/update docs

- Add `verda update` command: downloads binary from GitHub Releases
  - `verda update` — update to latest
  - `verda update --version v1.0.0` — install specific version
  - `verda update --list` — list available versions
  - Cross-platform: detects OS/arch, extracts tar.gz or zip
  - Atomic binary replacement (temp file + rename)
  - No Go installation required
- Update README with update command docs and release link

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hi-lei

.gitignore

@@ -357,3 +357,8 @@ dist
 tmp/
 temp/
 tmp-test-*/
+
+# VHS generated output (keep .tape files, ignore output)
+vhs/*.gif
+vhs/*.mp4
+vhs/*.webm

README.md

@@ -54,6 +54,18 @@ go install github.com/verda-cloud/verda-cli/cmd/verda@latest
 verda version
 ```
 
+### Update to latest version
+
+```bash
+verda update
+```
+
+Or install a specific version:
+
+```bash
+verda update --version v1.0.0
+```
+
 ## Getting Started
 
 ### 1. Configure credentials
@@ -102,6 +114,7 @@ Resource Commands:
 
 Other Commands:
   settings          Manage CLI settings
+  update            Update Verda CLI to latest or specific version
   version           Print version information
 ```
 
@@ -138,6 +151,14 @@ Other Commands:
 
 Available themes: `default`, `dracula`, `catppuccin`, `catppuccin-latte`, `nord`, `tokyonight`, `github-light`, `solarized-light`
 
+### Update
+
+| Command | Description |
+|---------|-------------|
+| `verda update` | Update to the latest version |
+| `verda update --version v1.0.0` | Install a specific version (upgrade or downgrade) |
+| `verda update --list` | List available versions |
+
 ### Auth
 
 | Command | Description |

internal/verda-cli/cmd/cmd.go

@@ -10,6 +10,7 @@ import (
 	"github/verda-cloud/verda-cli/internal/verda-cli/cmd/settings"
 	"github/verda-cloud/verda-cli/internal/verda-cli/cmd/sshkey"
 	"github/verda-cloud/verda-cli/internal/verda-cli/cmd/startupscript"
+	"github/verda-cloud/verda-cli/internal/verda-cli/cmd/update"
 	cmdutil "github/verda-cloud/verda-cli/internal/verda-cli/cmd/util"
 	"github/verda-cloud/verda-cli/internal/verda-cli/cmd/version"
 	"github/verda-cloud/verda-cli/internal/verda-cli/cmd/vm"
@@ -77,6 +78,7 @@ func NewRootCommand(ioStreams cmdutil.IOStreams) *cobra.Command {
 			Message: "Other Commands:",
 			Commands: []*cobra.Command{
 				settings.NewCmdSettings(f, ioStreams),
+				update.NewCmdUpdate(f, ioStreams),
 				version.NewCmdVersion(f, ioStreams),
 			},
 		},