Remove the remaining ZenHub references

[CodeMongerrr]

#87 took ZenHub out of the Python, but a few references survived in the shell scripts, the README and .gitignore. This clears out what was left.

What was still there:

• gen-schema.sh still introspected the ZenHub GraphQL API and ran sgqlc-codegen to produce zenhub_schema.py. Nothing imports that module any more, so it was generating a file no code reads — and it needs a ZENHUB_TOKEN file to get that far.
• gen-dag.sh still passed SHOW_EPICS=true and ZENHUB_TOKEN to zcash-issue-dag.py. Neither is read any more — the script only looks at GITHUB_TOKEN, DAG_VIEW, TERMINATE_AT, ONLY_INCLUDE, INCLUDE_FINISHED, PRUNE_FINISHED and SHOW_MILESTONES. Without a ZENHUB_TOKEN file sitting in the working directory it also prints cat: ZENHUB_TOKEN: No such file or directory once per view. It's harmless — the assignment doesn't trip set -e, so the loop still finishes — but it's a confusing thing to hit when you're setting the repo up for the first time.
• README.md asked for two token files and documented SHOW_EPICS, so a new contributor would go off and create a ZenHub token they don't actually need.
• .gitignore still ignored zenhub_schema.json, zenhub_schema.py and ZENHUB_TOKEN.
• One stale comment in helpers/github.py about "graph nodes from ZenHub" — those come from GitHub now.

I checked that nothing reads SHOW_EPICS or ZENHUB_TOKEN before pulling them out, and grep -ri zenhub now comes back empty across the tree. bash -n passes on both scripts.

One thing I noticed but deliberately left out of this PR, happy to send it separately if it'd be useful: the README lists seven DAG_VIEW values, but REPO_SETS in helpers/github.py currently has fourteen — zallet, ecc, zf-frost, zf-devops, zcashd-deprecation, sprout-deprecation and transparent-deprecation don't get a mention. TERMINATE_AT, ONLY_INCLUDE and PRUNE_FINISHED aren't documented either. Felt like a separate change from the ZenHub cleanup.