feat: add Claude Code marketplace plugin support

Add complete marketplace plugin configuration and documentation for GSD,
enabling installation via Claude Code's plugin marketplace.

Changes:
- Add .claude-plugin/marketplace.json and plugin.json manifests
- Add marketplace installation instructions to README.md
- Update all command files with marketplace tool permissions
- Add plugin conversion design and implementation documentation
- Update .gitignore for Claude Code plugin artifacts

Users can now install via:
  /plugin marketplace add glittercowboy/get-shit-done
  /plugin install get-shit-done@get-shit-done

.claude-plugin/plugin.json

@@ -0,0 +1,31 @@
+{
+  "name": "get-shit-done",
+  "version": "1.3.20",
+  "description": "A meta-prompting, context engineering and spec-driven development system for Claude Code by TÂCHES.",
+  "author": {
+    "name": "TÂCHES"
+  },
+  "license": "MIT",
+  "repository": "https://github.com/glittercowboy/get-shit-done",
+  "commands": [
+    "./commands/gsd/new-project.md",
+    "./commands/gsd/create-roadmap.md",
+    "./commands/gsd/map-codebase.md",
+    "./commands/gsd/plan-phase.md",
+    "./commands/gsd/execute-plan.md",
+    "./commands/gsd/progress.md",
+    "./commands/gsd/complete-milestone.md",
+    "./commands/gsd/discuss-milestone.md",
+    "./commands/gsd/new-milestone.md",
+    "./commands/gsd/add-phase.md",
+    "./commands/gsd/insert-phase.md",
+    "./commands/gsd/discuss-phase.md",
+    "./commands/gsd/research-phase.md",
+    "./commands/gsd/list-phase-assumptions.md",
+    "./commands/gsd/pause-work.md",
+    "./commands/gsd/resume-work.md",
+    "./commands/gsd/consider-issues.md",
+    "./commands/gsd/help.md",
+    "./commands/gsd/auto.md"
+  ]
+}