Merge Dev - Add derive subcommand

[aliroberts]

Summary

Adds weco run derive <run-id> — create a new run that branches off an existing run's step, inheriting its context and continuing optimization from there.

CLI

weco run derive [--from-step best|run-best|] [-n STEPS]
[-i INSTRUCTIONS] [--api-key provider=key ...]
[--output rich|plain]

• --from-step accepts best (lineage-best, default), run-best, or an integer step; integers resolve to a node UUID via list_nodes before the request.
• -i/--additional-instructions takes inline text or a file path; omit to inherit the parent run's instructions.
• --output plain emits bracket-tagged text for machine consumers; rich (default) drives the interactive panel.

Implementation notes

• New WecoClient.derive_run → POST /runs/{run_id}/derive, plus get_lineage for resolving derived-from metadata.
• weco/ui.py split into a package: base.py (Protocol + UIState), live.py, plain.py. OptimizationUI.on_init now takes an optional derived_from dict so the header can show the parent step and metric.
• Heartbeat loop factored into a heartbeat(run_id, auth_headers) context manager to remove the Event/start/try/finally/join boilerplate from handlers.
• _dispatch_run_subcommand now honours a bool return from handlers that drive an optimization loop, so derive can exit non-zero on failure without touching read-only handlers.