docs(examples): add multi-agent notepad demo

[zredlined]

Summary

Adds a runnable example that demonstrates a multi-agent shared notepad: isolated Codex agents run in separate OpenShell sandboxes, write markdown notes to a scoped GitHub path, and a synthesis agent reads those notes to produce a final summary.

The demo uses Codex as the agent runtime and GitHub as the durable notes backend, but the user-facing concept is a shared notepad for sandboxed agent coordination rather than a general-purpose agent memory system.

Related Issue

No issue yet. This PR captures the working demo for review.

Changes

• Adds examples/multi-agent-notepad/demo.sh to launch parallel worker sandboxes and a synthesis sandbox.
• Adds a scoped GitHub REST policy template for writes under runs/<run-id>/**.
• Adds worker and synthesis prompt templates for a markdown notepad workflow.
• Adds a concise README with setup, repo safety guidance, credential model, expected output, and network policy summary.
• Keeps sandbox uploads limited to the demo runner and prompt payload so workers do not inherit host logs or previous worker artifacts.

Testing

• mise run pre-commit passes locally; attempted after installing missing local tools, but the Rust test sandbox_create_keeps_sandbox_with_forwarding failed because port 8080 is already in use by the local gateway/Docker process
• bash -n examples/multi-agent-notepad/demo.sh
• Extracted generated sandbox runner and ran bash -n
• Parsed policy.template.yaml with Ruby YAML
• git diff --check -- examples/multi-agent-notepad
• markdownlint-cli2 over the new README and prompt templates, after isolating unrelated untracked local docs
• mise run pre-commit partial pass before the local port conflict: license check, markdownlint, Mermaid lint, Python format/lint/typecheck/tests, Helm lint, Rust format/check all completed successfully
• E2E smoke test with DEMO_AGENT_COUNT=2 against disposable private repo zredlined/openshell-codex-memory-demo-20260427-102752
• Verified generated agent-1.md, agent-2.md, and summary.md exist through the GitHub Contents API
• Verified temporary demo sandboxes and providers were cleaned up

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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