Add root library package with Config, Run and ProcessFile

Expose the core mark functionality as an importable Go library.
Library users can now import github.com/kovetskiy/mark and call:

  err := mark.Run(mark.Config{
      BaseURL:  "https://confluence.example.com",
      Username: "user",
      Password: "token",
      Files:    "docs/**/*.md",
      Features: []string{"mermaid", "mention"},
  })

The new package provides:
- Config struct: all options decoupled from the CLI framework
- Run(config Config) error: process all files matching Config.Files
- ProcessFile(file, api, config): process a single markdown file

Also moves the CLI entry point to cmd/mark/main.go following standard
Go convention for projects that serve as both a library and a binary.

Fixes a pre-existing nil-pointer dereference on meta.Attachments,
meta.Layout and related fields when using --target-url with a pageId
(meta was nil in that code path).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

cmd/mark/main.go

@@ -0,0 +1,39 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"github.com/kovetskiy/mark/util"
+	"github.com/reconquest/pkg/log"
+	"github.com/urfave/cli/v3"
+)
+
+var (
+	version = "dev"
+	commit  = "none"
+)
+
+const (
+	usage       = "A tool for updating Atlassian Confluence pages from markdown."
+	description = `Mark is a tool to update Atlassian Confluence pages from markdown. Documentation is available here: https://github.com/kovetskiy/mark`
+)
+
+func main() {
+	cmd := &cli.Command{
+		Name:                  "mark",
+		Usage:                 usage,
+		Description:           description,
+		Version:               fmt.Sprintf("%s@%s", version, commit),
+		Flags:                 util.Flags,
+		EnableShellCompletion: true,
+		HideHelpCommand:       true,
+		Before:                util.CheckFlags,
+		Action:                util.RunMark,
+	}
+
+	if err := cmd.Run(context.TODO(), os.Args); err != nil {
+		log.Fatal(err)
+	}
+}