Add Go's standard binary directory to the Zsh PATH

[barbatdev]

Problem

The shared Zsh profile prepends $HOME/.local/bin but does not include Go's executable installation directory. With an empty GOBIN, go install writes binaries to the first GOPATH entry plus /bin (commonly $HOME/go/bin). The installation succeeds, but the shell can keep resolving an older duplicate from $HOME/.local/bin or fail to find the newly installed binary.

Reproduction

1. Use the profile with go env GOBIN empty and the default GOPATH.
2. Install a CLI normally, for example:

   go install github.com/gentleman-programming/gentle-ai/v2/cmd/gentle-ai@v2.4.0-rc.1

3. Run command -v gentle-ai and gentle-ai version.

Expected behavior

The standard go install ...@version command should make the newly installed binary discoverable without requiring a per-command GOBIN override. The Go binary directory should take precedence over stale duplicates in generic user-local directories.

Current behavior

go install writes the requested version under the default Go binary directory, while the profile prioritizes $HOME/.local/bin; the active command can therefore remain on an older version.

Requested outcome

• Resolve the effective Go executable directory from non-empty GOBIN, otherwise from the first GOPATH entry plus /bin.
• Add it to the shared Zsh PATH with deterministic precedence.
• Avoid duplicate PATH entries and preserve compatibility when Go is unavailable.
• Add a shell smoke check covering the default GOPATH case.

Environment

• Zsh profile managed by this repository
• Go using its default GOPATH-based binary destination
• mise activation also participates in PATH ordering