Two blockers from the #844 review:
1. Rename nomad/jobs/forgejo.hcl → nomad/jobs/forgejo.nomad.hcl to match
the convention documented in nomad/AGENTS.md:38 (*.nomad.hcl suffix).
First jobspec sets the pattern for all future ones; keeps any glob-
based tooling over nomad/jobs/*.nomad.hcl working.
2. Add a dedicated `nomad-job-validate` step to .woodpecker/nomad-validate.yml.
`nomad config validate` (step 1) parses agent configs only — it rejects
jobspec HCL as "unknown block 'job'". `nomad job validate` is the
correct offline validator for jobspec HCL. Per the Hashicorp docs it
does not require a running agent (exit 0 clean, 1 on syntax/semantic
error). New jobspecs will add an explicit line alongside forgejo's,
matching step 1's enumeration pattern and this file's "no-ad-hoc-steps"
principle.
Also updated the file header comment and the pipeline's top-of-file step
index to reflect the new step ordering (2. nomad-job-validate inserted;
old 2-4 renumbered to 3-5).
Refs: #840 (S1.1), PR #844
