Use go.mod ignore directive instead of postinstall

[jakebailey]

Before 1.25, we needed a postinstall to force Go to ignore node_modules. Now, we can just stick it in go.mod.

This also means we can now drop the _ prefix on the dirs, but it's probably too late to change that.

[Copilot]

Pull Request Overview

This PR modernizes the project's Go module configuration by replacing a workaround for ignoring directories with the native go.mod ignore directive, available since Go 1.25.

• Removes the postinstall script that manually created a go.mod file in node_modules to prevent Go from scanning it
• Adds an ignore directive to go.mod that explicitly lists directories Go should ignore during module operations

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
package.json	Removes the postinstall script that was creating the Go workaround
go.mod	Adds ignore directive listing directories to exclude from Go module operations
Herebyfile.mjs	Removes the postinstall task implementation that wrote go.mod to node_modules

[jakebailey]

When specified without ./, this means "ignore node_modules at any level", which I think is correct in general.