feat(cli): add explicit external goal mode

[moortekweb-art]

Summary

• add a dedicated external goal path to CLI without altering the existing verified do flow:
  • external-do for free-form managed-mode starts
  • external-status and external-watch companion commands
• pass explicit managed dispatch parameters through external-do:
  • mode, safe area, checks, execution profile, supervision, monitor, JSON receipt
• provide readable guidance in README for the new flow
• add focused tests for command dispatch and backend error paths

Validation

• uv run --extra test pytest -q tests/test_external_goal_cli.py
• uv run --extra test ruff check agentic_harness/cli.py tests/test_external_goal_cli.py README.md

cd /private/var/folders/84/plrtcvr91rd9wbyk7tkmk6nw0000gp/T/agentic-harness-pr.XXXXXX.hNSMpJpJ1K
uv run --extra test pytest -q tests/test_external_goal_cli.py
uv run --extra test ruff check agentic_harness/cli.py tests/test_external_goal_cli.py README.md