zuul: drop dead image-push post jobs and rename tox jobs

[ideaship]

Summary

Removes the two post-pipeline image-push jobs that have failed with
RETRY_LIMIT on every run since the 2025-09-24 cfg-generics →
generics rename, and drops the now-stale cfg- prefix from the
surviving tox jobs.

Two separate commits, one logical change each.

zuul: drop dead image-push post jobs

Zuul encrypts each secret with the project's own RSA key.
SECRET_CFG_GENERICS was last encrypted against the key for project
osism/cfg-generics. After the repo was renamed, Zuul sees project
osism/generics (a different key) and can no longer decrypt the
secret. Both push jobs pass-to-parent that secret, so decryption
fails on the executor before any playbook runs; the failure is
retryable, so each attempt dies in ~1s with no logs and exhausts the
retries → RETRY_LIMIT.

The breakage went unnoticed for ~8 months because the jobs were
redundant even when green: the image Dockerfiles git checkout a fixed
tag pinned in osism/release, not generics main, so a post-on-main
rebuild bakes in the pinned tag rather than the triggering commit. The
real :latest refresh comes from the image repos' own post jobs and
their nightly periodic builds, which these jobs merely duplicated.

Removes the two job definitions, their post: entries, and the
now-unused SECRET_CFG_GENERICS secret.

zuul: rename cfg-generics-tox jobs to generics-tox

Pure rename dropping the stale cfg- prefix from the five tox jobs and
their references in the check and periodic-daily pipelines. No
external references to the old names exist in zuul-config/zuul-jobs.

Context

Root-cause triage:

• Rename repository from cfg-generics -> generics #560

Verification

• .zuul.yaml parses and passes yamllint after each commit.
• Confirmed nothing outside this file references the removed or renamed
  jobs (zuul-config, zuul-jobs, and this repo all checked).

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