feat: add developer-essentials plugin with 8 essential skills (#104)

Add new developer-essentials plugin containing 8 high-value skills that
address critical gaps in the existing skill coverage:

Skills Added:
- git-advanced-workflows: Advanced Git techniques (rebase, cherry-pick, bisect, worktrees, reflog)
- sql-optimization-patterns: SQL query optimization, indexing, and EXPLAIN analysis
- error-handling-patterns: Multi-language error handling strategies and patterns
- code-review-excellence: Effective code review practices and constructive feedback
- e2e-testing-patterns: End-to-end testing with Playwright and Cypress
- auth-implementation-patterns: Authentication/authorization with JWT, OAuth2, RBAC
- debugging-strategies: Systematic debugging techniques and profiling tools
- monorepo-management: Monorepo setup and management with Turborepo, Nx, pnpm

These skills were identified through comprehensive research of:
- Existing 50+ skills across 14 plugins in the project
- Anthropic's official skills repository
- Gap analysis revealing missing coverage in general developer workflows

Changes:
- Added plugins/developer-essentials/ with 8 new SKILL.md files
- Updated .claude-plugin/marketplace.json to register new plugin
- Updated docs/agent-skills.md (47→55 skills, 14→15 plugins)

Each skill follows Anthropic's Agent Skills Specification with proper
YAML frontmatter, progressive disclosure, and clear activation triggers.

🤖 Generated with Claude Code (https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -1951,6 +1951,43 @@
         "./skills/shellcheck-configuration/SKILL.md",
         "./skills/bats-testing-patterns/SKILL.md"
       ]
+    },
+    {
+      "name": "developer-essentials",
+      "source": "./plugins/developer-essentials",
+      "description": "Essential developer skills including Git workflows, SQL optimization, error handling, code review, E2E testing, authentication, debugging, and monorepo management",
+      "version": "1.0.0",
+      "author": {
+        "name": "Seth Hobson",
+        "url": "https://github.com/wshobson"
+      },
+      "homepage": "https://github.com/wshobson/agents",
+      "repository": "https://github.com/wshobson/agents",
+      "license": "MIT",
+      "keywords": [
+        "git",
+        "sql",
+        "debugging",
+        "testing",
+        "authentication",
+        "code-review",
+        "monorepo",
+        "essential"
+      ],
+      "category": "development",
+      "strict": false,
+      "commands": [],
+      "agents": [],
+      "skills": [
+        "./skills/git-advanced-workflows",
+        "./skills/sql-optimization-patterns",
+        "./skills/error-handling-patterns",
+        "./skills/code-review-excellence",
+        "./skills/e2e-testing-patterns",
+        "./skills/auth-implementation-patterns",
+        "./skills/debugging-strategies",
+        "./skills/monorepo-management"
+      ]
     }
   ]
 }