docs: document conditional -v test rerun pattern in REPO_POLICIES.md (#21)

## Summary

Adds the conditional verbose test rerun pattern as a policy recommendation in REPO_POLICIES.md.

Per sneak's request from [sneak/chat PR #82](sneak/chat#82): document the pattern where `make test` runs tests without `-v` first, then automatically reruns with `-v` on failure for full diagnostic output.

## Changes

**`prompts/REPO_POLICIES.md`** (root `REPO_POLICIES.md` is a symlink to this):
- Added new policy bullet after the `make test` timeout rule
- Explains the rationale: clean CI/Docker build logs on success, full verbose output on failure
- Includes a generic shell pattern template
- Includes concrete Go and Python examples
- Documents that `exit 1` ensures the target always fails after a rerun (the rerun is solely for diagnostic output)
- Updated `last_modified` from 2026-03-12 to 2026-03-18

## The Pattern

```makefile
test:
	@go test -timeout 30s -race -cover ./... || \
		{ echo "--- Rerunning with -v for details ---"; \
		  go test -timeout 30s -race -v ./...; exit 1; }
```

- **On success**: concise package summaries only, no per-test noise
- **On failure**: automatic verbose rerun shows every test case and assertion
- **Always fails**: `exit 1` ensures the build fails regardless of second run's exit code

closes #20

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #21
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

prompts/REPO_POLICIES.md

@@ -1,6 +1,6 @@
 ---
 title: Repository Policies
-last_modified: 2026-03-12
+last_modified: 2026-03-18
 ---
 
 This document covers repository structure, tooling, and workflow standards. Code
@@ -149,6 +149,42 @@ style conventions are in separate documents:
 - `make test` must complete in under 20 seconds. Add a 30-second timeout in the
   Makefile.
 
+- **`make test` should use the conditional verbose rerun pattern.** Run tests
+  without `-v` (verbose) first. If tests fail, automatically rerun with `-v` to
+  show full output. This keeps CI logs and `docker build` output clean on
+  success (just package/suite summaries) while providing full diagnostic detail
+  on failure (every test case, every assertion). The general shell pattern:
+
+    ```makefile
+    test:
+    	@<test-command> || \
+    		{ echo "--- Rerunning with -v for details ---"; \
+    		  <test-command-with-v>; exit 1; }
+    ```
+
+    Go example:
+
+    ```makefile
+    test:
+    	@go test -timeout 30s -race -cover ./... || \
+    		{ echo "--- Rerunning with -v for details ---"; \
+    		  go test -timeout 30s -race -v ./...; exit 1; }
+    ```
+
+    Python example:
+
+    ```makefile
+    test:
+    	@python -m pytest || \
+    		{ echo "--- Rerunning with -v for details ---"; \
+    		  python -m pytest -v; exit 1; }
+    ```
+
+    The `exit 1` ensures the target always fails after a rerun — the first run
+    already proved the tests are broken, so the build must not pass even if a
+    flaky test happens to succeed on the second attempt. The rerun exists solely
+    for diagnostic output.
+
 - Docker builds must complete in under 5 minutes.
 
 - `make check` must not modify any files in the repo. Tests may use temporary