Splits the 2103-line dashboard.py into two modules. Pure data
structures (QueuedProposal), discovery helpers (discover_pending,
discover_active_agents), derived-value helpers (_is_recent,
_approval_status, _format_agent_row, _detail_lines, etc.), and
argv-builder helpers (_build_split_pane_argv, _build_respawn_pane_argv,
_build_resume_argv_with_fallback, _agent_runtime_args) all move to
dashboard_model.py. The curses TUI, $EDITOR integration, tmux
subprocess flows, and action handlers (approve, reject,
operator_edit_routes, operator_edit_allowlist) remain in dashboard.py,
which re-imports everything from dashboard_model so existing callers and
tests are unaffected.
Adds tests/unit/test_dashboard_model.py covering _approval_status,
_proposed_payload_label, and _suffix_for_tool — three helpers that had
no prior coverage. All 894 unit tests pass.
Closes#158
