docs: add 11 new documentation files across all categories
New reference docs:
- agents/ref-btmsg.md: inter-agent messaging schema and CLI
- agents/ref-bttask.md: kanban task board operations
- providers/ref-providers.md: Claude/Codex/Ollama/Aider comparison
- config/ref-settings.md: (already committed)
New guides:
- contributing/dual-repo-workflow.md: community vs commercial repos
- plugins/guide-developing.md: Web Worker sandbox API and publishing
New pro docs:
- pro/features/knowledge-base.md: persistent memory + symbol graph
- pro/features/git-integration.md: context injection + branch policy
- pro/marketplace/README.md: 13 plugins catalog
Split files:
- architecture/data-model.md: from architecture.md (schemas, layout)
- production/hardening.md: from production.md (supervisor, sandbox, WAL)
- production/features.md: from production.md (FTS5, plugins, secrets, audit)
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