docs: update go/README.md to reflect single-module structure

[mesutoezdil]

Closes #1670.

The go.work file was removed in #1485 but go/README.md still described the directory as a Go workspace with three separate modules. It now points to a single go.mod, so updated the title, intro, section heading, table header, and directory tree to match.

[Copilot]

Pull request overview

Updates go/README.md to reflect the post-#1485 reality that go/ is now a single Go module (no go.work), and adjusts headings/table/tree language accordingly.

Changes:

• Rename the document from “Go Workspace” to “Go” and update the intro to describe a single module.
• Rename “Modules” → “Packages” and update the table headers to match.
• Update the directory tree to reference go.mod instead of go.work.

Comments suppressed due to low confidence (1)

go/README.md:30

• This tree still describes api/, core/, and adk/ as “module” directories (e.g., “Shared types module”), which conflicts with the updated “single Go module” framing. Update these comments to refer to directories/package areas rather than separate modules.

├── go.mod               # Single Go module file
├── Makefile              # Unified build targets
├── Dockerfile            # Shared multi-stage Docker build
│
├── api/                  # Shared types module

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[Copilot]

This section labels api, core, and adk as “Package” and uses paths like go/api/, which aren’t actual Go import paths and may be read as single-package directories. To reduce confusion, consider renaming the column to “Directory/Component” (or similar) and/or adjusting paths to match the directory tree below (api/, core/, adk/) or providing example import paths under the module (github.com/kagent-dev/kagent/go/api/...).