add installation of gen-commands

[gregmankes]

What was changed

Why?

Checklist

1. Closes

2. How was this tested:

3. Any docs updates needed?

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Note

Low Risk
Build tooling changes only; main risk is CI/local build breakage due to relying on git clone/network availability and the upstream temporalio/cli repo layout.

Overview
Makes code generation self-contained by adding a Makefile install target that temporarily clones temporalio/cli and go installs cmd/gen-commands, and making gen depend on it.

Removes the go.mod tool pin for gen-commands and trims now-unused module dependencies, and updates .gitignore to exclude the temporary cli/ clone directory.

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