Refactor CLI to use Cobra command framework

[zhubert]

Summary

Replaces the manual flag parsing in main.go with the Cobra command framework, improving CLI structure, help generation, and maintainability.

Changes

• Add spf13/cobra dependency for command-line parsing
• Create cmd/ package with modular command structure:
  • root.go - Root command with TUI startup and global --debug flag
  • clean.go - New unified clean command (replaces --clear and --prune)
  • mcp_server.go - Internal MCP server subcommand
  • demo.go - Demo generation subcommands
• Consolidate --clear and --prune flags into single plural clean command with confirmation prompt
• Add -y/--yes flag to skip confirmation in clean command
• Remove --check-prereqs flag (prerequisites still validated on TUI startup)
• Simplify main.go to just call cmd.Execute()
• Update CLAUDE.md documentation with new CLI usage

Test plan

• Run go test ./... to verify all tests pass
• Run plural --help to verify help output
• Run plural --version to verify version display
• Run plural clean and verify confirmation prompt appears
• Run plural clean -y to verify skip confirmation works
• Run plural demo list to verify demo subcommand works
• Run plural to verify TUI starts normally

[github-actions]

🔍 Deadcode Analysis

✅ No unreachable functions detected. Nice work keeping the codebase clean!