Bring conductor resume to flag parity with conductor run

[jrob5756]

Problem

conductor resume was missing several flags that exist on conductor run, making the recovery story confusing and broken in important cases. The most painful gap: a workflow started with --web or --web-bg could not be resumed with a dashboard, so users lost visibility exactly when something had just gone wrong.

What's added to resume

Flag	Purpose
--provider / -p	Runtime provider override
--metadata / -m	CLI metadata merged on top of YAML metadata
--web	Start a real-time web dashboard
--web-port	Dashboard port (0 = auto-select)
--web-bg	Fork a detached process running resume + dashboard, print URL, exit

--web and --web-bg are mutually exclusive (matching run).

Intentionally not mirrored

Flag	Why
--input / -i	Inputs are restored from the checkpoint context
--workspace-instructions, --instructions	instructions_preamble is persisted in the checkpoint
--dry-run	Incompatible with executing from a saved point

Implementation notes

• resume_workflow_async() now wires up the same WorkflowEventEmitter, EventLogSubscriber, ConsoleEventSubscriber, WebDashboard lifecycle, and RunContext as run_workflow_async().
• Stop-signal handling refactored into a shared _execute_with_stop_signal helper used by both _run_with_stop_signal and the new _resume_with_stop_signal.
• New launch_background_resume() in bg_runner.py forks a detached conductor resume subprocess and writes a PID file so conductor stop can find it.

Dashboard behavior on resume

Documented in the docstring: the dashboard only shows events from the resumed agent forward. Events from agents that completed before the checkpoint were emitted in the original process and are not replayed.

Future-proofing parity

Added a new Run / Resume Parity subsection to AGENTS.md (mirroring the existing Provider Parity style) listing the parity rule, the flags that must stay aligned, and the flags intentionally skipped — so future contributors keep them in sync.

Tests

10 new cases in tests/test_cli/test_resume_command.py:

• --provider / -m flags pass through to resume_workflow_async
• Malformed --metadata is rejected
• --web + --web-port flags pass through
• --web + --web-bg mutex error
• --web-bg dispatches to launch_background_resume with workflow path or --from checkpoint
• Direct unit tests of launch_background_resume command construction (subcommand, --from, port, provider, metadata) and ValueError when neither workflow_path nor checkpoint_path is given

Verification

• uv run ruff check src tests — clean
• uv run ruff format --check src tests — clean
• Full test suite: 2382 passed, 9 skipped (no regressions)

Example

# Start a workflow with the dashboard
conductor run workflow.yaml --web-bg

# It crashes — now resume it WITH the dashboard
conductor resume workflow.yaml --web-bg