Document tart exec usage and examples

[fkorotkov-oai]

Summary

• Expand tart exec --help with the macOS and guest-agent prerequisites and a link to the guest-agent documentation.
• Explain network-free execution, guest-user privileges, direct command versus shell execution, and guest exit-code propagation.
• Add copy-pasteable examples for running a command, running a shell command, opening an interactive shell, and redirecting a local script into the guest.

Why

The exec help previously provided only a short overview and did not explain common usage or where to find guest-agent documentation.

Validation

• swift build --build-system swiftbuild --product tart --scratch-path /Users/fkorotkov/code/tart/.build in a clean worktree: passed.
• Freshly built tart exec --help: verified that all four examples and the guest-agent documentation link render correctly.
• env DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer swift test --build-system swiftbuild --scratch-path /Users/fkorotkov/code/tart/.build: passed; 56 tests, zero failures, four Docker-dependent registry tests skipped because Docker is not installed.
• git diff --cached --check: passed.
• Independent review agent: no actionable findings.