docs: split inbox test plans into case files

docs/tests/inbox/README.md

@@ -9,15 +9,16 @@ It complements automated Go tests. The goal is not to restate implementation det
 ## Directory Rules
 
 - one folder per command or shared area
-- one `README.md` per folder
-- no one-file-per-case sprawl
+- each folder keeps a `README.md` entrypoint
+- command folders use `README.md` as an index only
+- each command test case lives in its own Markdown file named after the case slug
 - no numeric test IDs
-- each case is identified by `path + case slug`
+- each command case is identified by its concrete file path
 
-Recommended case heading pattern:
+Case file naming pattern:
 
-```md
-  ## case: send-rejects-invalid-payload-json
+```text
+<case-slug>.md
 ```
 
 ## Authoring Principles
@@ -48,7 +49,7 @@ Unless a case says otherwise:
 - `README.md`: global conventions and glossary
 - `_shared/README.md`: reusable fixtures, JSON assertions, exit codes, payload rules
 - `workflows/README.md`: cross-command end-to-end scenarios
-- per-command folders: command-specific cases and edge conditions
+- per-command folders: command-specific index `README.md` files plus one case document per test case
 
 ## Glossary
 
@@ -66,5 +67,5 @@ The current best executable reference is [internal/cli/inbox/integration_test.go
 When this Markdown plan is expanded:
 
 - prefer matching an existing automated scenario first
-- record any additional manual-only contract coverage explicitly in the relevant command document
+- record any additional manual-only contract coverage explicitly in the relevant command case file and keep the folder index synchronized
 - keep `docs/tests/inbox/ROADMAP.md` synchronized with authored files and case slugs