fix: address review feedback on persona feature

MAJOR fixes:
- Remove external YAML dependency (github.com/goccy/go-yaml)
  Per project convention: Go standard library only, zero dependencies.
  Convert all persona files from YAML to JSON format.
- Fix TestValidateWorkspacePath error expectation
  Go 1.21+ filepath.Join normalizes absolute paths differently.

MINOR fixes:
- Remove custom contains helper in persona_test.go (use strings.Contains)
- Add Unicode-safe CapitalizeFirst function for header titles
- ListBuiltinPersonas returns empty slice instead of nil on error
- Fix test comment about filepath.Join behavior

Documentation:
- Update README to reflect JSON-only persona format
- Update design doc with note about JSON decision
- Fix action.yml description for persona-file input

README.md

@@ -380,38 +380,32 @@ Each persona posts independently with its own sentinel, so reviews don't interfe
 
 ### Custom Personas
 
-Create a YAML file with your domain-specific review focus:
+Create a JSON file with your domain-specific review focus:
 
-```yaml
-# .review/personas/trading.yaml
-name: trading
-display_name: Trading Domain Expert
-
-identity: |
-  You are a trading systems expert reviewing code for correctness.
-  
-  Your expertise:
-  - Order lifecycle and state machines
-  - Fill handling and partial fills
-  - Position tracking and P&L calculations
-  - Event sourcing invariants
-
-focus:
-  - Order state machine correctness
-  - Fill handling edge cases (partial, overfill)
-  - Position and P&L calculation accuracy
-  - Event replay determinism
-  - Decimal precision for money
-
-ignore:
-  - Code style
-  - General performance
-  - Documentation formatting
-
-severity:
-  major: Bugs that cause incorrect positions, fills, or money calculations
-  minor: Edge cases that could cause issues under unusual conditions
-  nit: Clarity improvements for domain logic
+```json
+// .review/personas/trading.json
+{
+  "name": "trading",
+  "display_name": "Trading Domain Expert",
+  "identity": "You are a trading systems expert reviewing code for correctness.\n\nYour expertise:\n- Order lifecycle and state machines\n- Fill handling and partial fills\n- Position tracking and P&L calculations\n- Event sourcing invariants",
+  "focus": [
+    "Order state machine correctness",
+    "Fill handling edge cases (partial, overfill)",
+    "Position and P&L calculation accuracy",
+    "Event replay determinism",
+    "Decimal precision for money"
+  ],
+  "ignore": [
+    "Code style",
+    "General performance",
+    "Documentation formatting"
+  ],
+  "severity": {
+    "major": "Bugs that cause incorrect positions, fills, or money calculations",
+    "minor": "Edge cases that could cause issues under unusual conditions",
+    "nit": "Clarity improvements for domain logic"
+  }
+}
 ```
 
 Use it in CI:
@@ -420,18 +414,17 @@ Use it in CI:
 - uses: rodin/review-bot/.gitea/actions/review@v1
   with:
     reviewer-name: trading
-    persona-file: .review/personas/trading.yaml
+    persona-file: .review/personas/trading.json
     ...
 ```
 
-JSON format is also supported for backwards compatibility.
 
 ### Persona vs system-prompt-file
 
 | Feature | `persona` / `persona-file` | `system-prompt-file` |
 |---------|---------------------------|----------------------|
 | Replaces base prompt | Yes | No (appends) |
-| Structured format | Yes (YAML/JSON) | No (freeform) |
+| Structured format | Yes (JSON) | No (freeform) |
 | Focus/ignore lists | Yes | Manual |
 | Severity calibration | Yes | Manual |
 | Header display name | Yes | No |