Add Claude Code marketplace plugin support

- Add .claude-plugin/marketplace.json and plugin.json manifests
- Use ${CLAUDE_PLUGIN_ROOT:-$HOME/.claude/skills/last30days} for script path
- Plugin install: uses CLAUDE_PLUGIN_ROOT when set
- Manual install: falls back to ~/.claude/skills/last30days (no breakage)
- SKILL.md stays at root, plugin.json points skills to "./"

Users can now install via:
  claude plugin marketplace add mvanhorn/last30days-skill
  claude plugin install last30days@last30days

Inspired by PR #1 (galligan) — lighter approach without repo restructure.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

SKILL.md

@@ -58,7 +58,7 @@ This text MUST appear before you call any tools. It confirms to the user that yo
 
 **Step 1: Run the research script**
 ```bash
-python3 ~/.claude/skills/last30days/scripts/last30days.py "$ARGUMENTS" --emit=compact 2>&1
+python3 "${CLAUDE_PLUGIN_ROOT:-$HOME/.claude/skills/last30days}/scripts/last30days.py" "$ARGUMENTS" --emit=compact 2>&1
 ```
 
 The script will automatically: