Add agent mode

[mmanciop]

Detect common AI coding agents and change the behavior of the CLI to:

• Set default output to JSON
• Use JSON output for --help
• Structured errors in JSON
• Skip confirmation steps (--force)

The detection is based on process environment variables, but can be forced on with the --agent-mode global boolean flag and turned on and off with the DASH0_AGENT_MODE=0|1|false|true environment variable.