chore: add .coderabbit.yaml pinning Docstring Coverage to warning (#582)

## Summary
Adds `.coderabbit.yaml` pinning CodeRabbit's Docstring Coverage
pre-merge check to `mode: warning` (threshold 80) and enabling the
docstrings finishing touch.

## Why
This repo's effective `main` rules require the `CodeRabbit` status
context (measured 2026-10-01 via `rules/branches/main`), so an
`error`-mode Docstring Coverage check would make PRs here unmergeable at
today's baseline. The level is pinned explicitly (an estate decision)
rather than left to the vendor default.

## Verification
- Key paths `reviews.pre_merge_checks.docstrings.{mode,threshold}` and
`reviews.finishing_touches.docstrings.enabled` checked against the
fetched https://coderabbit.ai/integrations/schema.v2.json (structural
check: keys present in schema, `mode` in enum, `threshold` a number in
[0,100], `enabled` boolean). No path differences from the intended
layout.
- SPDX header `MPL-2.0` on line 1.
- Plain YAML; parses with yq.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01Hw7qg3u9PAP6b2oKyVTSVC

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Chores**
* Docstring checks now issue warnings when the configured threshold is
not met, and docstring finishing touches are enabled.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: hyperpolymath

.coderabbit.yaml

@@ -0,0 +1,16 @@
+# SPDX-License-Identifier: MPL-2.0
+# CodeRabbit configuration. Schema: https://coderabbit.ai/integrations/schema.v2.json
+#
+# Docstring Coverage pre-merge check is pinned to `mode: warning` as an explicit
+# estate decision rather than left to the vendor default. Some estate repos'
+# rulesets require the `CodeRabbit` status context; an `error` mode would make
+# them unmergeable at today's docstring-coverage baseline. Raise it per repo only
+# once that repo's coverage clears the threshold.
+reviews:
+  pre_merge_checks:
+    docstrings:
+      mode: warning
+      threshold: 80
+  finishing_touches:
+    docstrings:
+      enabled: true