refactor: comprehensive code quality improvements

- Add logger to all exporters for consistent error handling
- Add Course.Validate() method with ValidationError type
- Create shared test fixtures in internal/testdata
- Refactor config passing: NewArticulateParser now takes *config.Config
- Add NewLoggerFromConfig() factory for logger creation
- Add context cancellation with signal handling (SIGINT/SIGTERM)
- Add godoc examples for loggers and exporters
- Add HTML exporter error path tests
- Add fuzz tests for parser and HTMLCleaner
- Fix markdown output: trim whitespace, collapse blank lines
- Remove test output files from repo

.opencode/opencode.jsonc

@@ -0,0 +1,25 @@
+{
+	"$schema": "https://opencode.ai/config.json",
+	"lsp": {
+		"golangci-lint": {
+			"command": ["golangci-lint-langserver"],
+			"extensions": [".go"],
+			"initialization": {
+				"command": [
+					"golangci-lint",
+					"run",
+					"--output.json.path",
+					"stdout",
+					"--show-stats=false",
+					"--issues-exit-code=1",
+				],
+			},
+		},
+	},
+	"formatter": {
+		"dprint": {
+			"command": ["dprint", "fmt", "$FILE"],
+			"extensions": [".go", ".md", ".Dockerfile", ".Dockerfile.dev", ".md"],
+		},
+	},
+}