Step 7: Add remove command to stop tracking files.

Implements Garden.Remove() which unregisters paths from the manifest,
plus unit tests and the CLI wiring via cobra.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

garden/remove.go

@@ -0,0 +1,38 @@
+package garden
+
+import (
+	"fmt"
+	"path/filepath"
+)
+
+// Remove stops tracking the given paths. Each path is resolved to absolute
+// form, converted to a tilde path, and removed from the manifest. An error
+// is returned if any path is not currently tracked.
+func (g *Garden) Remove(paths []string) error {
+	for _, p := range paths {
+		abs, err := filepath.Abs(p)
+		if err != nil {
+			return fmt.Errorf("resolving path %s: %w", p, err)
+		}
+
+		tilded := toTildePath(abs)
+
+		if g.findEntry(tilded) == nil {
+			return fmt.Errorf("not tracking %s", tilded)
+		}
+
+		filtered := g.manifest.Files[:0]
+		for _, e := range g.manifest.Files {
+			if e.Path != tilded {
+				filtered = append(filtered, e)
+			}
+		}
+		g.manifest.Files = filtered
+	}
+
+	if err := g.manifest.Save(g.manifestPath); err != nil {
+		return fmt.Errorf("saving manifest: %w", err)
+	}
+
+	return nil
+}