feat: scaffold new command and generator orchestration

[dDevAhmed]

Summary

Implements CLI-023 by registering stellarforge new <project-name> and defining a thin, testable project-generation orchestration boundary.

Included

• new <project-name> command registration
• generator input/result contracts
• explicit validation → destination planning → template selection → generation → optional install → optional Git initialization stage ordering
• dependency-injected services for isolated testing
• default scaffold behavior that performs no filesystem mutation and fails safely before unimplemented generation stages
• command-level and orchestration-level tests
• root help/registration coverage for new

Security

• validation is always the first orchestration stage
• later stages are not invoked when validation fails
• no filesystem mutation is implemented in this issue
• no arbitrary remote template loading
• no plugin system
• no shell/process execution

Scope

Project-name/destination hardening is intentionally deferred to CLI-024, controlled template registry work to CLI-025, and actual template copying/substitution to CLI-026.

Closes #26