docs: explain GitHub operating model and token economics

[Stolyarovmn]

Summary

This PR expands the README with the rationale for using GitHub as part of the Hybrid Agentic OS rather than keeping the whole control plane local-only.

It explains:

• why GitHub is used as a durable shared control plane for framework development;
• which responsibilities move from local runtime files into GitHub-native surfaces;
• which pieces remain local by design;
• how the split saves LLM tokens by moving deterministic coordination into Issues, PRs, Projects, Actions, labels and templates.

Why this is better

The local runtime is still the fast execution layer: STATE.md, targets/*.local.md, tasks/TASK-*.md, inbox/, outbox/, tmux sessions and credentials remain machine-local.

GitHub is used where it is stronger and cheaper than agent context: durable IDs, timelines, review comments, PR diffs, status checks, labels, project views and issue forms. That means agents spend fewer tokens rediscovering state from chat or scanning local files, and more tokens on judgment, implementation and audit.

Verification

• git diff --check
• bash -n *.sh lib/runtime.sh
• ./smoke-runtime.sh