Add API_POLICIES.md (closes #137)

prompts/API_POLICIES.md states sneak's rule from
sneak/routewatch#199: a value in an API
response is never a string with a label or unit in it. A native value
is sent as that type, with its unit in the field name.

The list at the top of REPO_POLICIES.md and the two code style
commands in the README now name the new file.

Model: opus-5-5

prompts/API_POLICIES.md

@@ -0,0 +1,20 @@
+---
+title: API Policies
+last_modified: 2026-10-27
+---
+
+A value in an API response is never a string with a label or unit in it. A value
+of a native type is sent as that type, and its unit goes in the field name, for
+example `_ms`.
+
+Wrong:
+
+```json
+{ "execution_time": "0 ms" }
+```
+
+Right:
+
+```json
+{ "execution_time_ms": 0 }
+```

prompts/REPO_POLICIES.md

@@ -1,6 +1,6 @@
 ---
 title: Repository Policies
-last_modified: 2026-10-26
+last_modified: 2026-10-27
 ---
 
 This document covers repository structure, tooling, and workflow standards. Code
@@ -12,6 +12,7 @@ style conventions are in separate documents:
 - [JavaScript](https://git.eeqj.de/sneak/prompts/raw/branch/main/prompts/CODE_STYLEGUIDE_JS.md)
 - [Python](https://git.eeqj.de/sneak/prompts/raw/branch/main/prompts/CODE_STYLEGUIDE_PYTHON.md)
 - [Go HTTP Server Conventions](https://git.eeqj.de/sneak/prompts/raw/branch/main/prompts/GO_HTTP_SERVER_CONVENTIONS.md)
+- [API Policies](https://git.eeqj.de/sneak/prompts/raw/branch/main/prompts/API_POLICIES.md)
 
 ---