Document subprocess run results

docs/consumers/subprocess.md

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+# Using Notarius As A Subprocess
+
+Use this workflow when an orchestrator runs Notarius and consumes its published
+artifacts. The [CLI reference](../cli.md) owns invocation syntax and exit
+statuses, while the [run-result receipt](../integrations/run-result.md) and
+[Published JSON Output contract](../integrations/json-output.md) own the
+durable result formats.
+
+## Run And Check The Process
+
+Optionally preflight a selected configuration and pipeline before work starts:
+
+```sh
+notarius config validate --config /path/to/notarius.yml --pipeline pipeline-id
+```
+
+Invoke the run with explicit paths and machine-readable output. Capture
+standard output and standard error separately; do not combine them before
+processing the result.
+
+```sh
+notarius run pipeline-id \
+  --config /path/to/notarius.yml \
+  --input /path/to/source.json \
+  --output-dir /path/to/output-root \
+  --json
+```
+
+Use absolute paths for supplied input, configuration, output-root, and
+reference files. When a stable prompt session identifier or references are
+needed, pass the supported CLI flags. Supply credentials through Notarius's
+documented configuration and environment mechanisms, never as command-line
+arguments or generated secret-bearing configuration.
+
+Wait for the process before interpreting standard output. Only an exit status
+of 0 permits decoding the receipt. On a nonzero exit, retain standard error for
+diagnosis and ignore all standard-output bytes: a failed receipt write may have
+left a partial document.
+
+## Discover Required Artifacts
+
+Decode the successful receipt and accept the schema versions supported by the
+caller. Use its `output_directory` as the bundle root. For the production JSON
+output, resolve `index_file` under that root with a confinement check and reject
+an absolute path or a result that escapes the root.
+
+Read the resulting `index.json` and locate each artifact by `lane_id`, not by a
+guessed filename. Before decoding a selected payload, verify its descriptor's
+media type and schema identity against the relevant published artifact
+contract. The JSON bundle contract links to the available lane contracts.
+
+A zero exit status may still report rejected outputs, warnings, or absent
+lanes. The caller decides which lane IDs are required for its own work and
+which are optional; it should make that decision explicitly rather than infer
+failure from the receipt counts alone.
+
+## Preserve Provenance And Handle Data Carefully
+
+Keep the receipt with the published `manifest.json`, and retain
+`rejected.json` and `warnings.json` when review or later provenance requires
+them. Treat the input, output bundle, cache, debug bundle, and captured process
+logs as potentially sensitive data. Apply the caller's access controls and
+retention policy, and avoid copying secrets into arguments, logs, or
+provenance records.