WIP: V3 - Initial Cut of Modernized Go version

[AdamDrewsTR]

Solves #480

[AdamDrewsTR]

Version Architecture - Two Modes

GLOBAL Mode

 - Trigger: Using ~/.goenv/version (no local .go-version or go.mod)
 - Use case: General command-line work, not in a specific project
 - Resolution:
   - Version bin (~/.goenv/versions/1.25.4/bin/)
   - Version GOPATH (~/.goenv/versions/1.25.4/gopath/bin/)
   - Host bin (~/.goenv/hosts/darwin-arm64/gopath/bin/) ← Shared tools
 - When: Working in home directory, doing one-off commands, etc.

LOCAL Mode

 - Trigger: .go-version or go.mod in current directory or parent
 - Use case: Working on a specific project
 - Resolution:
   - Version bin (~/.goenv/versions/1.23.2/bin/)
   - Version GOPATH (~/.goenv/versions/1.23.2/gopath/bin/)
   - Host bin NOT checked ← Project isolation!
 - When: Inside any project directory with version file

Version Directory Sharing

Multiple projects using the same version share the same Go installation:

 Project A (.go-version: 1.23.2) ──┐
                                    ├──> ~/.goenv/versions/1.23.2/
 Project B (.go-version: 1.23.2) ──┘
 
 Same Go binary:      ~/.goenv/versions/1.23.2/bin/go
 Same version GOPATH: ~/.goenv/versions/1.23.2/gopath/bin/

This means:

 - ✅ Efficient: One Go installation per version, not per project
 - ✅ Shared tools: If you go install a tool in project A with 1.23.2, project B (also using 1.23.2) will see it
 - ✅ Version isolation: Projects using different versions (1.23.2 vs 1.24.0) have completely separate tool sets
 - ⚠️ Module cache: Shared via ~/.goenv/shared/go-mod (intentional, modules are version-agnostic source code)

The Consistency Rule

Everywhere in goenv now follows this standard:

 - exec: Uses resolver with version source ✅
 - which: Uses resolver with version source ✅  
 - shims: Will execute using the same resolution ✅
 - whence: Shows versions that have the tool (version-specific only) ✅

The ONLY exception is:

 - Host bin tools are ONLY accessible in GLOBAL mode

This gives you:

 - Predictable isolation when working on projects
 - Convenient sharing of global tools
 - Efficient storage (one Go installation per version)
 - Safe compatibility (tools match the Go version you're using)