Prepare v5.1.0: release notes and version bump

Add v5.1.0 release notes covering:
- Removals: legacy slash commands (/brainstorm, /execute-plan,
  /write-plan), skill Integration sections
- Worktree skills rewrite (PRI-974, PR #1121)
- Contributor guidelines for AI agents
- Codex plugin mirror tooling (PR #1165)
- OpenCode bootstrap caching (#1202)
- SDD pause-every-3-tasks fix; SDD integration test fixes
- Cursor Windows hook routing
- Gemini CLI subagent dispatch mapping
- Skill terminology cleanups
- Install docs (Factory Droid, Codex, quickstart links)

Bumps version 5.0.7 -> 5.1.0 across all declared files via
scripts/bump-version.sh; not yet tagged or released.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -9,7 +9,7 @@
     {
       "name": "superpowers",
       "description": "Core skills library for Claude Code: TDD, debugging, collaboration patterns, and proven techniques",
-      "version": "5.0.7",
+      "version": "5.1.0",
       "source": "./",
       "author": {
         "name": "Jesse Vincent",

.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "superpowers",
   "description": "Core skills library for Claude Code: TDD, debugging, collaboration patterns, and proven techniques",
-  "version": "5.0.7",
+  "version": "5.1.0",
   "author": {
     "name": "Jesse Vincent",
     "email": "jesse@fsck.com"