feat(cli): add JSON output to exec

[Whxuan0701]

TL;DR

Add structured JSON output to msb exec so agents and host-side orchestrators can consume exit status and captured output without parsing terminal text.

Description

• Add msb exec NAME --format json -- COMMAND for non-interactive execution.
• Return exit_code, success, stdout, and stderr in one JSON object.
• Preserve UTF-8 as text and encode arbitrary output bytes as Base64 with explicit encoding fields.
• Keep the guest exit code as the msb process exit code.
• Reject JSON output with --tty or --stream, and document the machine-readable contract.

Test Plan

• cargo fmt --all -- --check
• cargo test -p microsandbox-cli --lib in rust:1.97-bookworm with libcap-ng-dev (314 passed)
• cargo clippy -p microsandbox-cli --lib -- -D warnings in the same Linux container

Real microVM execution was not run because the available host is Intel macOS, while this project's local libkrun runtime does not support that host configuration.

[greptile-apps]

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A["msb exec --format json"] --> B{"Explicit or image-default command?"}
  B -->|Yes| C["Execute and capture output"]
  C --> D["Serialize JSON result"]
  B -->|No| E["Exit 0 before serialization"]

Loading

Prompt To Fix All With AI

### Issue 1
crates/cli/lib/commands/exec.rs:107-111
**JSON result skipped without command**

When `--format json` is used without an explicit command and the sandbox image has no default command, non-interactive resolution reaches the successful early-exit branch before serialization, causing the process to exit with status 0 while stdout contains no JSON result.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "feat(cli): add JSON output to exec" | Re-trigger Greptile}

[greptile-apps]

_{Reviews (2): Last reviewed commit: "fix(cli): preserve exec JSON output cont..." | Re-trigger Greptile}