[SPN-2931] Stop SDK regen from deleting .github/ tree

[drago1216]

Summary

PR #23 (auto/sdk-update) was deleting our entire `.github/` directory on every regen — `CODEOWNERS`, `generate.yml`, `publish-pypi.yml`, `publish-testpypi.yml`, and `sdk-update.yml` itself all showed as removed in the bot PR diff.

Root cause

`scripts/post_generate.py` did:

```python
for unwanted in [".gitlab-ci.yml", ".github", "test-requirements.txt"]:
...
elif path.is_dir():
shutil.rmtree(path)
```

That logic was written when `.github/` was 100% openapi-generator output (the Python template still writes a default `.github/workflows/python.yml`). Since then we've added our own workflows under `.github/`, but the cleanup kept indiscriminately rm-rf'ing the whole tree.

Fix

1. Add `.github/**` to `.openapi-generator-ignore` so the generator never writes there in the first place. The cleanup step then becomes a no-op for `.github/` regardless of what the upstream template emits.
2. Remove `.github` from `post_generate.py`'s unwanted list, with a comment so a future maintainer doesn't re-add it.

Verified

Ran `make generate` locally after the change. `.github/` contents are byte-identical to before; `git status` reports a clean working tree for the directory.

Test plan

• Re-trigger `sdk-update.yml` after merge; the bot PR should no longer show `.github/` deletions.

🤖 Generated with Claude Code