Implement functionality in order to generate report directly from the artifacts folder.

README.md

@@ -17,10 +17,30 @@ pointing at a module that was never put on the path.
 
 ## Running
 
+Two ways to say where the artifacts are.
+
+**Out of the store** — Postgres for the rows, the bucket for the files:
+
 ```
-report_generator --scan_id <uuid> [--path <file-or-directory>] --config config.json
+report_generator generate --scan_id <uuid> [--path <file-or-directory>] --config config.json
 ```
 
+**Out of a folder** — neither Postgres nor the bucket, just the artifacts as the pipeline exported
+them:
+
+```
+report_generator from-folder --artifacts ./sample [--path <file-or-directory>]
+```
+
+`--scan_id` is optional there: the pipeline writes its own id into the paths it records in
+`metadata.json`, so an export names its own run. Pass it for a folder assembled by hand, or one
+whose metadata is gone.
+
+The two produce the same document. Everything after the artifacts are in hand is shared code, and
+the sample export in `sample/` was checked against the report built from the store for the same
+scan: 17 of 17 pages identical. The tests cover the folder path; comparing the two needs a store
+and so is not among them.
+
 `--scan_id` takes either id a run is known by. A run is recorded under two: the **pipeline id**,
 which the GUI shows, cipherverifier names its working directories after, and archon's `core_scan`
 row is keyed by; and the **stored scan id**, minted when the artifacts are uploaded, which `scans`,