chore(stewardship): 🌱 add .grove.yaml declaration (#160)

## Summary

Add `.grove.yaml` stewardship declaration for Grove observation.
Declares intent, horizon, role, phase, steward, and consolidation
cadence.

## Test plan

- [ ] `.grove.yaml` is valid YAML with correct field values
- [ ] No other files affected

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

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: justapithecus

.grove.yaml

@@ -0,0 +1,10 @@
+grove:
+  intent: >-
+    Structured persistence framework for object storage,
+    bringing snapshots, metadata, and safe write semantics
+    without running a database.
+  horizon: "generational"
+  role: "library"
+  phase: "consolidating"
+  steward: "Andrew"
+  consolidation_interval_days: 90