Add research index for branch studies and skill

Author: Your Name

README.md

@@ -214,6 +214,7 @@ session marks a local boundary, so its child is analyzed as a local root.
 - [Codex CLI rollout parsing case study](docs/case-studies/codex-rollout-parser.md)
 - [Claude Code hook causality failure observation](examples/traces/failures/observations-claude-code-hooks.md)
 - [Runtime research notes](docs/research-notes/README.md)
+- [Research index](docs/research/README.md)
 - [Project principles](docs/project-principles.md)
 
 ---

README.zh-CN.md

@@ -210,6 +210,7 @@ causetrace compare <session_a> <session_b>
 - [Codex CLI rollout 解析案例](docs/case-studies/codex-rollout-parser.md)
 - [Claude Code hook 因果链失效观察](examples/traces/failures/observations-claude-code-hooks.md)
 - [运行时研究笔记](docs/research-notes/README.md)
+- [研究索引](docs/research/README.md)
 - [项目原则](docs/project-principles.md)
 
 ---

docs/research/README.md

@@ -0,0 +1,29 @@
+# Research Index
+
+This directory groups the research tracks and branch studies that sit alongside the main `causetrace` runtime-morphology work.
+
+## Active Research Tracks
+
+- [Phase 3A](phase3a/README.md)
+- [Phase 3B](phase3b/README.md)
+- [Phase 3C](phase3c/README.md)
+- [Phase 3D](phase3d/README.md)
+
+## Cross-project Branch Studies
+
+- [Cross-project Prompt Morphology Study](branches/cross_project_prompt_morphology/README.md)
+  - public research branch
+  - v0.1 / v0.2 complete
+  - apply-first phase active
+
+## Related Skill
+
+- [prompt-routing-skill](https://github.com/milkoor/prompt-routing-skill)
+  - derived from the cross-project prompt morphology work
+  - routes tasks to `minimal_prompt`, `human_structured_prompt`, or `expanded_constrained_prompt`
+  - pairs with `causetrace` by selecting prompt posture first and measuring the resulting trace morphology here
+
+## Boundary
+
+Branch studies and skills may inform hypotheses and workflow choices, but they do not change the `causetrace` core boundary by themselves.
+