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

cmd/review-bot/main_test.go

@@ -103,11 +103,13 @@ func TestValidateWorkspacePath(t *testing.T) {
 			errMatch:  "resolves outside workspace",
 		},
 		{
-			name:      "absolute path gets normalized to relative",
+			name:      "absolute path normalized to workspace-relative",
 			workspace: tmpDir,
 			path:      "/etc/passwd",
 			wantErr:   true,
-			errMatch:  "failed to resolve", // filepath.Join strips leading / making it <workspace>/etc/passwd which doesn't exist
+			// Go 1.21+ filepath.Join normalizes absolute paths: Join("/tmp/x", "/etc/passwd")
+			// becomes "/tmp/x/etc/passwd", which is within workspace but doesn't exist.
+			errMatch: "failed to resolve",
 		},
 		{
 			name:      "nonexistent file",
@@ -152,7 +154,6 @@ func TestValidateWorkspacePath(t *testing.T) {
 	}
 }
 
-
 func makeReview(id int64, login, state string, stale bool, body string) gitea.Review {
 	r := gitea.Review{
 		ID:    id,
@@ -164,7 +165,6 @@ func makeReview(id int64, login, state string, stale bool, body string) gitea.Re
 	return r
 }
 
-
 func TestBuildSupersededBody(t *testing.T) {
 	original := "# Review\n\nLooks good.\n\n<!-- review-bot:sonnet -->"
 	sentinel := "<!-- review-bot:sonnet -->"
@@ -734,8 +734,8 @@ func TestExtractSentinelName_EdgeCases(t *testing.T) {
 		{"<!-- review-bot:sonnet --> rest", "sonnet"},
 		{"<!-- review-bot:gpt-review --> rest", "gpt-review"},
 		{"no sentinel here", "unknown"},
-		{"<!-- review-bot:", "unknown"},       // prefix but no suffix
-		{"prefix <!-- review-bot:abc --> end", "abc"}, // embedded in text
+		{"<!-- review-bot:", "unknown"},                   // prefix but no suffix
+		{"prefix <!-- review-bot:abc --> end", "abc"},     // embedded in text
 	}
 
 	for _, tc := range tests {