fix: resolve golangci-lint CI failures, simplify CI, tidy docs (#29)

* fix(lint): resolve golangci-lint failures and modernize string handling

CI's golangci-lint (v2.12.2) job was failing with 14 issues, which blocked
the dependent test job. This addresses all of them:

- goconst: extract repeated string literals into constants
  - format aliases ("md", "word", "htm") in the exporter factory
  - "section" lesson type shared by markdown and HTML exporters
  - default Articulate Rise base URL and host in the parser
  - reuse existing itemType* constants in the markdown switch
- staticcheck (QF1012): replace buf.WriteString(fmt.Sprintf(...)) with
  fmt.Fprintf(...) in the markdown exporter

Also drop the hardcoded `go: "1.24"` from .golangci.yml so the target Go
version is autodetected from go.mod.

* ci: drop test matrix, run a single Go version from go.mod

The test job ran a 1.24.x/1.25.x matrix, but go.mod requires go 1.25.0, so the
1.24 entry just auto-downloaded the 1.25 toolchain and tested the same thing
twice. Replace the matrix with a single job that sources its Go version from
go.mod via go-version-file, and reference the resolved version through the
setup-go step output in summaries, artifact names, and Codecov flags.

* docs: remove emojis from README

* ci: pin modernize tool to gopls v0.21.x for Go 1.25 compatibility

The autofix workflow's `task modernize` step installed the modernize
analyzer from gopls@latest, which as of v0.22.0 requires Go 1.26. The
project targets Go 1.25 (go.mod) and CI runs with GOTOOLCHAIN=local, so
the install failed. Pin to the v0.21.x line, which supports Go >= 1.25.

* ci: keep modernize@latest, fetch its toolchain via GOTOOLCHAIN=auto

Replaces the earlier v0.21.0 pin. The modernize analyzer (gopls v0.22+)
requires Go 1.26, which the project doesn't target yet — and bumping the
module to 1.26 isn't viable because the current golangci-lint release is
built with Go 1.25 and refuses to lint a newer target. Instead, let the
modernize task fetch the toolchain it needs on demand via GOTOOLCHAIN=auto
(setup-go pins GOTOOLCHAIN=local in CI), so we stay on the latest analyzer
without touching the module's Go version.

Also bump golang.org/x/image v0.34.0 -> v0.42.0 via `go get -u ./...`.

* ci: set GOTOOLCHAIN=auto inline for the modernize task

A task-level env: entry does not override GOTOOLCHAIN when setup-go has
already exported GOTOOLCHAIN=local job-wide, so the autofix job still
failed. Set GOTOOLCHAIN=auto inline on the modernize command itself, which
reliably overrides the inherited value and lets Go fetch the toolchain the
modernize analyzer requires.

* ci: bump to Go 1.26 and address PR review feedback

The autofix failure is fixed properly by moving to Go 1.26 instead of the
GOTOOLCHAIN workaround. The official golangci-lint v2.12.2 binary is built
with go1.26.2 and lints a Go 1.26 target fine (the earlier "not ready"
claim was from a locally go-installed binary compiled with Go 1.25), and
the Dockerfiles already use golang:1.26-alpine, so this also aligns the
module with the images.

- go.mod: go 1.25.0 -> 1.26.0, drop the toolchain pin (keeps the lint
  target at the go directive).
- Taskfile: revert modernize to plain modernize@latest; on Go 1.26 the
  GOTOOLCHAIN dance and its comments are unnecessary.
- ci.yml: pass the resolved Go version through an env var (GO_VERSION)
  instead of interpolating steps.setup-go.outputs.go-version directly into
  shell scripts (script-injection hygiene); grant the dependency-review job
  pull-requests: write so it can post its summary.
- parser.go: derive defaultBaseURL from riseHost instead of duplicating the
  host string; drop the redundant per-const comments.
- .golangci.yml: remove the redundant go-version comment.
- regenerate internal/exporters/output.docx.

internal/services/parser.go

@@ -15,6 +15,12 @@ import (
 	"github.com/kjanat/articulate-parser/internal/models"
 )
 
+// Default endpoint configuration for the Articulate Rise API.
+const (
+	riseHost       = "rise.articulate.com"
+	defaultBaseURL = "https://" + riseHost
+)
+
 // shareIDRegex is compiled once at package init for extracting share IDs from URIs.
 var shareIDRegex = regexp.MustCompile(`/share/([a-zA-Z0-9_-]+)`)
 
@@ -37,7 +43,7 @@ func NewArticulateParser(logger interfaces.Logger, baseURL string, timeout time.
 		logger = NewNoOpLogger()
 	}
 	if baseURL == "" {
-		baseURL = "https://rise.articulate.com"
+		baseURL = defaultBaseURL
 	}
 	if timeout == 0 {
 		timeout = 30 * time.Second
@@ -132,7 +138,7 @@ func (p *ArticulateParser) extractShareID(uri string) (string, error) {
 	}
 
 	// Validate that it's an Articulate Rise domain
-	if parsedURL.Host != "rise.articulate.com" {
+	if parsedURL.Host != riseHost {
 		return "", fmt.Errorf("invalid domain for Articulate Rise URI: %s", parsedURL.Host)
 	}