feat: switch execute to orchestrator API

[AlephNotation]

Summary

Switches vers execute from direct SSH to the orchestrator's exec streaming API.

Execute via orchestrator API

vers execute now uses POST /api/v1/vm/{id}/exec/stream by default. Commands flow through the vsock agent inside the VM, which means they automatically inherit /etc/environment (env vars).

vers execute <vm> ls -la /
vers execute --ssh <vm> echo "legacy path"   # direct SSH fallback
vers execute -w /app <vm> npm start           # working directory support

Key changes:

• Streams NDJSON output (stdout/stderr) in real time
• Propagates remote exit codes to the CLI process
• --ssh flag preserves legacy SSH behavior
• SetInterspersed(false) so command flags like -la pass through correctly
• --workdir / -w for remote working directory

Files changed

File	Change
cmd/execute.go	API by default, --ssh fallback, --workdir, flag passthrough
internal/handlers/execute.go	Dual path: API streaming + SSH fallback
internal/services/vm/exec.go	New — orchestrator exec stream client
internal/presenters/execute_types.go	Added ExitCode to view
cmd/env.go	Minor help text cleanup