Install · scope the skill to skills/hallmark/ (fixes #9)

The skills CLI copies everything under the directory containing SKILL.md.
With SKILL.md at the repo root, installs were pulling in docs/, site/,
package.json, README, ROADMAP, vercel.json, LICENSE. Moving the skill
into skills/hallmark/ scopes the install to just SKILL.md + references/.

The CLI's priority search hits skills/ first, so `npx skills add
nutlope/hallmark` is unchanged. Updated README install paths,
package.json (entry/references/files), and the internal links that
crossed the new boundary.

package.json

@@ -18,12 +18,11 @@
   "license": "MIT",
   "type": "module",
   "files": [
-    "SKILL.md",
-    "references"
+    "skills"
   ],
   "skill": {
-    "entry": "SKILL.md",
-    "references": "references",
+    "entry": "skills/hallmark/SKILL.md",
+    "references": "skills/hallmark/references",
     "harnesses": [
       "claude-code",
       "cursor",