refactor(exporter): rewrite HTML exporter to use Go templates

Replaces the manual string-building implementation of the HTML exporter with a more robust and maintainable solution using Go's `html/template` package. This improves readability, security, and separation of concerns.

- HTML structure and CSS styles are moved into their own files and embedded into the binary using `go:embed`.
- A new data preparation layer adapts the course model for the template, simplifying rendering logic.
- Tests are updated to reflect the new implementation, removing obsolete test cases for the old string-building methods.

Additionally, this commit:
- Adds an `AGENTS.md` file with development and contribution guidelines.
- Updates `.golangci.yml` to allow standard Go patterns for interface package naming.

AGENTS.md

@@ -0,0 +1,56 @@
+# Agent Guidelines for articulate-parser
+
+## Build/Test Commands
+- **Build**: `task build` or `go build -o bin/articulate-parser main.go`
+- **Run tests**: `task test` or `go test -race -timeout 5m ./...`
+- **Run single test**: `go test -v -race -run ^TestName$ ./path/to/package`
+- **Test with coverage**: 
+  - `task test:coverage` or 
+  - `go test -race -coverprofile=coverage/coverage.out -covermode=atomic ./...`
+- **Lint**: `task lint` (runs vet, fmt check, staticcheck, golangci-lint)
+- **Format**: `task fmt` or `gofmt -s -w .`
+- **CI checks**: `task ci` (deps, lint, test with coverage, build)
+
+## Code Style Guidelines
+
+### Imports
+- Use `goimports` with local prefix: `github.com/kjanat/articulate-parser`
+- Order: stdlib, external, internal packages
+- Group related imports together
+
+### Formatting
+- Use `gofmt -s` (simplify) and `gofumpt` with extra rules
+- Function length: max 100 lines, 50 statements
+- Cyclomatic complexity: max 15
+- Cognitive complexity: max 20
+
+### Types & Naming
+- Use interface-based design (see `internal/interfaces/`)
+- Export types/functions with clear godoc comments ending with period
+- Use descriptive names: `ArticulateParser`, `MarkdownExporter`
+- Receiver names: short (1-2 chars), consistent per type
+
+### Error Handling
+- Always wrap errors with context: `fmt.Errorf("operation failed: %w", err)`
+- Use `%w` verb for error wrapping to preserve error chain
+- Check all error returns (enforced by `errcheck`)
+- Document error handling rationale in defer blocks when ignoring close errors
+
+### Comments
+- All exported types/functions require godoc comments
+- End sentences with periods (`godot` linter enforced)
+- Mark known issues with TODO/FIXME/HACK/BUG/XXX
+
+### Security
+- Use `#nosec` with justification for deliberate security exceptions (G304 for CLI file paths, G306 for export file permissions)
+- Run `gosec` and `govulncheck` for security audits
+
+### Testing
+- Enable race detection: `-race` flag
+- Use table-driven tests where applicable
+- Mark test helpers with `t.Helper()`
+- Benchmarks in `*_bench_test.go`, examples in `*_example_test.go`
+
+### Dependencies
+- Minimal external dependencies (currently: go-docx, golang.org/x/net, golang.org/x/text)
+- Run `task deps:tidy` after adding/removing dependencies