docs: Add local lessons learned backup system

- Create docs/project-lessons-learned/ for local lesson storage
- Add INDEX.md with lesson template and index table
- Document Phase 4 dbt test syntax deprecation lesson
- Update CLAUDE.md with backup method when Wiki.js unavailable

This provides a fallback for capturing lessons learned while
Wiki.js integration is being configured.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/project-lessons-learned/INDEX.md

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+# Project Lessons Learned
+
+This folder contains lessons learned from sprints and development work. These lessons help prevent repeating mistakes and capture valuable insights.
+
+**Note:** This is a temporary local backup while Wiki.js integration is being configured. Once Wiki.js is ready, lessons will be migrated there for better searchability.
+
+---
+
+## Lessons Index
+
+| Date | Sprint/Phase | Title | Tags |
+|------|--------------|-------|------|
+| 2026-01-16 | Phase 4 | [dbt Test Syntax Deprecation](./phase-4-dbt-test-syntax.md) | dbt, testing, yaml, deprecation |
+
+---
+
+## How to Use
+
+### When Starting a Sprint
+1. Review relevant lessons in this folder before implementation
+2. Search by tags or keywords to find applicable insights
+3. Apply prevention strategies from past lessons
+
+### When Closing a Sprint
+1. Document any significant lessons learned
+2. Use the template below
+3. Add entry to the index table above
+
+---
+
+## Lesson Template
+
+```markdown
+# [Sprint/Phase] - [Lesson Title]
+
+## Context
+[What were you trying to do?]
+
+## Problem
+[What went wrong or what insight emerged?]
+
+## Solution
+[How did you solve it?]
+
+## Prevention
+[How can this be avoided in future sprints?]
+
+## Tags
+[Comma-separated tags for search]
+```

docs/project-lessons-learned/phase-4-dbt-test-syntax.md

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+# Phase 4 - dbt Test Syntax Deprecation
+
+## Context
+Implementing dbt mart models with `accepted_values` tests for tier columns (safety_tier, income_quintile, amenity_tier) that should only contain values 1-5.
+
+## Problem
+dbt 1.9+ introduced a deprecation warning for generic test arguments. The old syntax:
+
+```yaml
+tests:
+  - accepted_values:
+      values: [1, 2, 3, 4, 5]
+```
+
+Produces deprecation warnings:
+```
+MissingArgumentsPropertyInGenericTestDeprecation: Arguments to generic tests should be nested under the `arguments` property.
+```
+
+## Solution
+Nest test arguments under the `arguments` property:
+
+```yaml
+tests:
+  - accepted_values:
+      arguments:
+        values: [1, 2, 3, 4, 5]
+```
+
+This applies to all generic tests with arguments, not just `accepted_values`.
+
+## Prevention
+- When writing dbt schema YAML files, always use the `arguments:` nesting for generic tests
+- Run `dbt parse --no-partial-parse` to catch all deprecation warnings before they become errors
+- Check dbt changelog when upgrading versions for breaking changes to test syntax
+
+## Tags
+dbt, testing, yaml, deprecation, syntax, schema