docs: add research README and simplify docs index hierarchy

Each docs subdirectory now owns its own README index, matching the
pattern guide/ already followed. The top-level docs/README.md points
to subdirectories instead of reaching through to individual files.

Author: hakula139

docs/README.md

@@ -1,16 +1,9 @@
-# Documentation Index
+# Documentation
 
-## User Guide
+## Contents
 
-See [`guide/`](guide/) for user-facing documentation: [quickstart](guide/quickstart.md), [configuration](guide/configuration.md), and [instruction files](guide/instructions.md).
-
-## Internal
-
-Research findings, architecture notes, and project status.
-
-| Document                                                       | Description                                                            |
-| -------------------------------------------------------------- | ---------------------------------------------------------------------- |
-| [roadmap.md](roadmap.md)                                       | Project status: working features, current focus, planned phases        |
-| [research/anthropic-api.md](research/anthropic-api.md)         | Anthropic API auth: OAuth flow, required headers, system prompt prefix |
-| [research/extended-thinking.md](research/extended-thinking.md) | Extended thinking: content block types, signatures, round-tripping     |
-| [research/system-prompt.md](research/system-prompt.md)         | System prompt architecture: section assembly, CLAUDE.md, caching       |
+| Directory / File           | Description                                            |
+| -------------------------- | ------------------------------------------------------ |
+| [`guide/`](guide/)         | User-facing documentation: quickstart, config, howtos  |
+| [`research/`](research/)   | Architecture research and API reference notes          |
+| [`roadmap.md`](roadmap.md) | Project status: working features, current focus, plans |

docs/research/README.md

@@ -0,0 +1,9 @@
+# Research Notes
+
+Architecture research and API reference notes for oxide-code development.
+
+| Document                                  | Description                                                            |
+| ----------------------------------------- | ---------------------------------------------------------------------- |
+| [Anthropic API](anthropic-api.md)         | Anthropic API auth: OAuth flow, required headers, system prompt prefix |
+| [Extended Thinking](extended-thinking.md) | Extended thinking: content block types, signatures, round-tripping     |
+| [System Prompt](system-prompt.md)         | System prompt architecture: section assembly, CLAUDE.md, caching       |