docs(readme): Orca integration section + orchestrator emphasis

[choiyounggi]

Docs-only. Foregrounds the orchestrator and the Orca integration:

• Intro gains a paragraph positioning orchestrate as the multi-session orchestrator over the same loop (dependency graph, ready-set scheduler, Orca-native supervision).
• New "Orca integration — supervision, not just spawning" section: Run/Task/Dispatch provenance, event-driven worker_done/ask/escalation waits with dead-runtime detection, the env-carrying orca-worker-start.sh composition (guardrails escalation contract, re-entry probe), and two-question liveness — plus the tmux fallback's file-based parity (ask-coordinator question channel, stall surfacing, send-prompt.sh keys, launch-time status pre-seed).
• Skills-table orchestrate row: adds the hardened tmux watch and per-role model selection.
• Wording fix: the substrate is offered at the task-split gate ("automatic" implied no gate).

EN + KO in sync. Repo topics updated alongside: orca, orchestration, multi-agent, loop-engineering, graph-engineering.