fix(shell): respect POSH_DISABLE_STREAMING at runtime

[JanDeDobbeleer]

Summary

This fixes the case where POSH_DISABLE_STREAMING=1 is set before initialization but the cached shell init still starts streaming on the live session.

What changed

• removed the generation-time config gate from src/config/config.go
• kept the live runtime guard in the generated shell scripts, so the env var is checked when the script actually initializes streaming
• added/updated shell tests for fish, zsh, cmd, and pwsh to cover the runtime guard behavior

Validation

• cd src && go test ./config ./shell/...

[github-actions]

📦 Release binary size report

Compares this PR's release-equivalent build against the latest published release, per OS (amd64).

OS	Baseline	This PR	Delta
darwin	14.18 MB	14.18 MB	-0.0 KB (+0%)
linux	13.84 MB	13.84 MB	-4.0 KB (-0.03%)
windows	14.36 MB	14.31 MB	-42.8 KB (-0.29%)

🎉 Binary size shrank on at least one platform.