refactor: rename forge -> graff across the codebase

[justrach]

Summary

The codebase still uses forge naming extensively — crate names (forge_domain, forge_app, forge_main, etc.), module prefixes, config paths, and internal references. This should be renamed to graff to match the product name.

Scope

• Crate names: forge_domain -> graff_domain, forge_app -> graff_app, etc.
• Module prefixes: ForgeWidget, ForgeApi, etc.
• Config paths: any forge-prefixed config directories or files
• Internal references: comments, docs, error messages
• Binary names: forge_main crate already produces graff binary, but the crate name itself is still forge_main

Approach

Large refactor, do in phases:

1. Rename crates (Cargo.toml workspace members + all use paths)
2. Rename public types and traits
3. Rename config paths and env vars (with migration for existing users)
4. Update docs and comments