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

.golangci.yml

@@ -1,6 +1,7 @@
+# yaml-language-server: $schema=https://golangci-lint.run/jsonschema/golangci.jsonschema.json
 # golangci-lint configuration for articulate-parser
 # https://golangci-lint.run/usage/configuration/
-
+---
 version: "2"
 
 # Options for analysis running
@@ -51,6 +52,7 @@ formatters:
     - gofmt
     - goimports
     - gofumpt
+  exclusions: { paths: [".zed"] }
   settings:
     # gofmt settings
     gofmt: