docs: restructure docs into diataxis (#121)

docs/explanation/index.md

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+# Explanation
+
+Explanation docs answer **“how does this work?”** and **“why is it designed this way?”**
+
+If you want step-by-step instructions, go to **[Tutorials](../tutorials/index.md)**.  
+If you want exact options and contracts, go to **[Reference](../reference/index.md)**.
+
+## How Takopi works end-to-end
+
+- Incoming Telegram message → resolve context (project/branch) → resolve resume token → select runner → stream events → render progress → send final + resume line.
+
+Start here:
+
+- [Architecture](architecture.md)
+
+## Routing, sessions, and continuation
+
+Takopi is stateless by default, but can provide “continuation” in multiple ways:
+
+- reply-to-continue (always available)
+- per-topic resume (Telegram forum topics)
+- per-chat sessions (auto-resume)
+
+- [Routing & sessions](routing-and-sessions.md)
+
+## Plugins and extensibility
+
+Takopi uses entrypoint-based plugins with lazy discovery so broken plugins don’t brick the CLI.
+
+- [Plugin system](plugin-system.md)
+
+## Codebase orientation
+
+If you’re making changes, this is the “map of the territory”:
+
+- [Module map](module-map.md)
+
+## Where to look for hard rules
+
+Explanation pages describe intent and tradeoffs. The *hard requirements* live in:
+
+- [Reference: Specification](../reference/specification.md)
+- [Reference: Plugin API](../reference/plugin-api.md)