fix: mask secrets on every path they leave a job (#1188)

A secret in a matrix value reached the log in the clear:

```yaml
strategy:
  matrix:
    include: "${{ github.token }}"
```

Chasing that one route is pointless, so this masks every sink a secret leaves a job by: the uploaded log rows and the on-disk `job.log`, both through one choke point in `appendLogRow`; the runner's own log, which is where planning errors like that one land with no job logger in reach; the job logger's stdout under debug logging; job summaries; job outputs; and the job name that becomes a container name.

Values the runner knows but the job never declared, the proxy password and the task token, are hidden the same way. Masks apply longest first, since `strings.Replacer` matches in argument order and one secret prefixing another would otherwise mask the prefix and print the rest.

### What changes for users

An output whose value carries a secret is skipped with a warning instead of sent, matching GitHub. Output that showed a secret now shows `***`. `ACTIONS_STEP_DEBUG` and `ACTIONS_RUNNER_DEBUG` are never masked, also matching GitHub, so an output of `true` still reaches the jobs that need it.

Each fix has a test that fails without it.

Reviewed-on: https://gitea.com/gitea/runner/pulls/1188
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

README.md

@@ -390,6 +390,10 @@ See **[docs/job-hooks.md](docs/job-hooks.md)** for the execution order, environm
 
 Set `log.job.dir` to a path and the runner writes a copy of every task's log there as `<start time>-task-<id>.log`: the rows exactly as Gitea received them, with the same secrets masked and the job's result on the last line. Off by default, and what Gitea shows does not change.
 
+#### Secret masking
+
+A job's secrets and its `::add-mask::` values are hidden from what the runner writes and uploads: the job log, the local copy above, job summaries, and the names of the containers it creates. A job output carrying one is skipped with a warning rather than sent masked, as GitHub does, so a downstream `needs.<job>.outputs.<name>` reading it is empty.
+
 `log.job.retention` (default `168h`) is how long a log is kept, expired ones being deleted as new tasks start, and `log.job.max_size` (default `1GB`) caps one log. Keep `retention` above `runner.timeout` so a long job cannot outlive its own log, and prefer local disk, the file is written while the job runs. Only the runner's own user can read it.
 
 ### Example Deployments