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>

CLAUDE.md

@@ -270,9 +270,9 @@ All scripts in `scripts/`:
 | Skill | Trigger | Purpose |
 |-------|---------|---------|
 | `/projman:sprint-plan` | New sprint or phase implementation | Architecture analysis + Gitea issue creation |
-| `/projman:sprint-start` | Beginning implementation work | Load lessons learned, start execution |
+| `/projman:sprint-start` | Beginning implementation work | Load lessons learned (Wiki.js or local), start execution |
 | `/projman:sprint-status` | Check progress | Review blockers and completion status |
-| `/projman:sprint-close` | Sprint completion | Capture lessons learned to Wiki.js |
+| `/projman:sprint-close` | Sprint completion | Capture lessons learned (Wiki.js or local backup) |
 
 ### Default Behavior
 
@@ -299,6 +299,24 @@ When user requests implementation work:
 **Wiki.js**:
 - `search_lessons`, `create_lesson`, `search_pages`, `get_page`
 
+### Lessons Learned (Backup Method)
+
+**When Wiki.js is unavailable**, use the local backup in `docs/project-lessons-learned/`:
+
+**At Sprint Start:**
+1. Review `docs/project-lessons-learned/INDEX.md` for relevant past lessons
+2. Search lesson files by tags/keywords before implementation
+3. Apply prevention strategies from applicable lessons
+
+**At Sprint Close:**
+1. Try Wiki.js `create_lesson` first
+2. If Wiki.js fails, create lesson in `docs/project-lessons-learned/`
+3. Use naming convention: `{phase-or-sprint}-{short-description}.md`
+4. Update `INDEX.md` with new entry
+5. Follow the lesson template in INDEX.md
+
+**Migration:** Once Wiki.js is configured, lessons will be migrated there for better searchability.
+
 ### Issue Structure
 
 Every Gitea issue should include:

docs/project-lessons-learned/INDEX.md

@@ -0,0 +1,50 @@
+# 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

@@ -0,0 +1,38 @@
+# 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