feat: add user flag for data plane operations

[vaaandark]

Add --user flag for data plane operations

Add --user flag to exec, file, and instance login commands to specify the user identity for sandbox data plane calls. Previously all calls were hardcoded to "user".

Changes

• New resolveUser() helper: --user flag > sandbox.default_user config > "user"
• exec: replace hardcoded User: "user" with resolveUser()
• file: pass User config to all 7 filesystem SDK calls (previously nil)
• instance login: forward --user to webshell Manager.Start()
• New [sandbox] config section with default_user option (env: AGS_SANDBOX_DEFAULT_USER)
• Update REPL completions, docs, and changelogs

Usage

ags exec -i <id> --user root "whoami"
ags file ls -i <id> --user root /root
ags instance login <id> --user root