refactor: streamline profiles and fix registry pluralization

Profile Changes:
- Reduce core profile from 15 to 9 components (true essentials only)
- Keep developer at 22 components (add back image-specialist + gemini)
- Remove unused subagents from core (reviewer, build-agent)
- Remove non-essential commands from core (test, context, prompt-enhancer)

Core Profile (9 components):
- Agents: task-manager, codebase-agent
- Subagents: tester, documentation, coder-agent
- Commands: commit
- Context: essential-patterns, project-context
- Config: env-example

Developer Profile (22 components):
- Everything in core plus quality tools and AI features
- Added: image-specialist, gemini tool
- Added: reviewer, build-agent, codebase-pattern-analyst
- Added: test, context, clean, optimize, prompt-enhancer commands

Bug Fixes:
- Fix jq error: 'Cannot iterate over null' for config components
- Add get_registry_key() helper to handle singular 'config' vs plural types
- Update all jq queries to use correct registry keys

Codebase Agent:
- Remove @image-specialist reference (not needed for core coding)
- Update subagent list to match actual usage
- Now lists: @task-manager, @tester, @documentation, @coder-agent

Documentation:
- Update README with correct component counts
- Update profile descriptions to be more accurate
- Clarify what each profile is best for

This makes 'core' truly minimal and 'developer' the recommended choice.