fix: add commands/last30days.md and remove skills/last30days-nux duplicate (#267)

Adds commands/last30days.md so /last30days registers as a Claude Code
slash command for plugin users. Users type /last30days and autocomplete
prefix-matches to the canonical /last30days:last30days form (same as
/ce:plan resolving to /compound-engineering:ce-plan).

Removes skills/last30days-nux/, a byte-identical duplicate of the root
SKILL.md that created confusing /last30days:last30days-nux autocomplete
entries via Claude Code's plugin namespacing. Root SKILL.md remains
the canonical skill source; natural-language skill-selector invocation
is unchanged.

Recovery for users on v3.0.4: /plugin update last30days then /reload-plugins.

Closes #239 (path-escape error was already fixed in v3.0.4 by dropping
the rogue 'skills' key; v3.0.5 adds the slash command on top).
Supersedes #257 (suggested './' -> '.' workaround is obsolete since
v3.0.4 dropped the 'skills' key entirely, matching ecosystem standard).

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

.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "last30days",
-  "version": "3.0.4",
+  "version": "3.0.5",
   "description": "Research any topic across Reddit, X, YouTube, TikTok, Instagram, Hacker News, Polymarket, GitHub, and 5+ more sources. AI agent scores by upvotes, likes, and real money - not editors.",
   "author": {
     "name": "Matt Van Horn",