fix: restore skills/ and .claude-plugin/ in plugin install tarball (#262)

v3.0.1 added .gitattributes rules that excluded both directories from
git archive output, shrinking the claude.ai .skill bundle. But Claude
Code's /plugin install fetches the SAME archive, so users installing
v3.0.1 or v3.0.2 received a tarball with no plugin manifest and no
skill files. Install appeared successful but the plugin was a useless
empty shell.

Proof:
  git archive v3.0.0 | grep 'skills/|\.claude-plugin/' | wc -l  # 8
  git archive v3.0.1 | grep 'skills/|\.claude-plugin/' | wc -l  # 0
  git archive v3.0.2 | grep 'skills/|\.claude-plugin/' | wc -l  # 0

No issue reports yet because:
 - Cached pre-v3.0.1 installs keep working (it's the new-install path
   that's broken)
 - The breakage is under 24 hours old
 - Users invoking the skill via natural language go through
   skill-selector rather than /last30days slash command

Also reverts v3.0.2's "skills": ["skills"] back to "./", the value
that shipped in every tag from v2.1.0 through v3.0.0. That change was
a misdiagnosis; the manifest wasn't in the tarball anyway so it had
no effect on user-visible installs.

Archive file count after fix: 97 (cap is 200, plenty of room).
Follow-up: move claude.ai-specific bundle exclusions into
scripts/build-skill.sh where they belong, rather than .gitattributes
which cannot distinguish between the two distribution channels.

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>

gemini-extension.json

@@ -1,6 +1,6 @@
 {
   "name": "last30days-skill",
-  "version": "3.0.2",
+  "version": "3.0.3",
   "description": "Research a topic from the last 30 days across Reddit, X, YouTube, TikTok, Instagram, Hacker News, Polymarket, and the web.",
   "settings": [
     {