docs: restructure docs into diataxis (#121)

docs/how-to/index.md

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+# How-to guides
+
+How-to guides are **goal-oriented recipes**. Pick the task you’re trying to accomplish and follow the steps.
+
+If you’re learning from scratch, start with **[Tutorials](../tutorials/index.md)**.  
+If you need exact options and defaults, use **[Reference](../reference/index.md)**.
+
+## Daily use
+
+- [Switch engines](switch-engines.md) (`/codex`, `/claude`, `/opencode`, `/pi`)
+- [Projects](projects.md) (register repos + run from anywhere)
+- [Worktrees](worktrees.md) (run work on `@branch` without switching your main checkout)
+- [Route by chat](route-by-chat.md) (dedicated chats per project)
+- [Topics](topics.md) (forum threads bound to repo/branch + auto-resume)
+- [Chat sessions](topics.md#chat-sessions) (auto-resume without replying)
+
+## Messaging extras
+
+- [Voice notes](voice-notes.md) (transcribe and run)
+- [File transfer](file-transfer.md) (`/file put` and `/file get`)
+- [Schedule tasks](schedule-tasks.md) (Telegram scheduled messages)
+
+## Extending Takopi
+
+- [Write a plugin](write-a-plugin.md) (engines, transports, commands)
+- [Add a runner](add-a-runner.md) (implement a new engine backend)
+- [Dev setup](dev-setup.md) (run from source, tests, linting, type checks)
+
+## Debugging and operations
+
+- [Troubleshooting](troubleshooting.md) (`--debug`, common gotchas, “why didn’t it route?”)
+
+## Not sure where to go?
+
+- If your question starts with “**How do I…**” → you’re in the right place.
+- If your question starts with “**What are the exact options / defaults?**” → go to **[Reference](../reference/index.md)**.
+- If your question starts with “**Why is it designed this way?**” → go to **[Explanation](../explanation/index.md)**.