Derive @bust/config types from the schema

[busticated]

Description

Config.get() returned a seven-member union for a key typed as plain string, so every call site had to coerce or cast the result and a typo'd key only surfaced at runtime. This threads the schema's shape through as a type parameter, so e.g. get('app.port') returns number, an enum format returns its literal union, and an undeclared key is a compile error.

How to Test

1. Clone and setup this branch locally (docs)
2. Run unit tests: npm test
3. Run end-to-end tests: npm run test:e2e
4. In a scratch file under packages/config/src/, call createConfig() with a schema using several formats, in your IDE hover the get() results, then run npm run typecheck

Outcome

All tests pass. Each get() call reports the type its format implies rather than SettingsValue, an undeclared key fails typecheck, and an untyped new Config() still accepts any string key.

Related / Discussions

N/A

Completeness

• PR opened 🎉
• Testing instructions have been provided
• Docs have been updated (npm run docs:build)
• Changes adhere to repo conventions
• Development How-To's have been provided
• Branch is rebased against target (typically main)