docs: document bundled mappers, jsonata: expressions and the reshape-in-the-mapper convention

The README never described mappers/ or incident-types/ although the
sync collects both. Documents the mapper file format, the jsonata:
prefix with $parseJson/$toTable, the singleton [] trap, and the
convention that new integrations reshape in the mapper rather than
pre-building display structures in scripts.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L1fWBoEKvAw3k78XDkpRQo

README.md

@@ -26,8 +26,12 @@ and walks the whole tree looking for files named exactly **`manifest.yaml`**.
 integrations/
 └── <integration-id>/
     ├── manifest.yaml          # required — the integration definition
-    └── scripts/               # optional — only for script-based commands
-        └── <command-id>.py
+    ├── scripts/               # optional — only for script-based commands
+    │   └── <command-id>.py
+    ├── mappers/               # optional — bundled mapper templates, one per command
+    │   └── <command-id>.yaml
+    └── incident-types/        # optional — bundled incident types
+        └── <anything>.yaml
 ```
 
 ---
@@ -132,6 +136,72 @@ Two rules the catalog applies consistently, because they are not obvious:
 
 ---
 
+## Bundled mappers (`mappers/<command-id>.yaml`)
+
+A mapper turns one raw element of a command's output into an incident: the
+fields Riposte shows and routes on, and an OCSF projection for entity
+indexing. Bundled mappers are registered as **read-only library templates**
+on install and **replaced on every update** — an operator's clones are never
+touched, which also means a clone never receives your fixes. Ship correct
+templates; let operators clone only to diverge on purpose.
+
+```yaml
+name: "My Product Investigations → OCSF"   # required — shown in the mapper library
+description: |
+  Free text. Say what one element looks like and what the mapper reads.
+
+# Incident fields. Target: title, severity, source, description, custom:<name>.
+field_mappings:
+  title: "title"                              # dotted path into the element
+  severity: "priority"                        # values run through toSeverity (CRITICAL→5 … LOW→2)
+  "custom:vendor_status": "status"            # lands in the declared custom field <name>
+
+# OCSF projection. source_path is full JSONata; ocsf_field is the destination.
+ocsf:
+  - { source_path: "rrn",          ocsf_field: "finding_info.uid" }
+  - { source_path: "created_time", ocsf_field: "finding_info.created_time" }
+```
+
+### Expressions: dotted paths and `jsonata:`
+
+A `field_mappings` expression is a dotted path (with an optional
+`cond ? then : else` ternary). Prefixed with **`jsonata:`**, it is evaluated
+by the full JSONata engine instead — the same one `ocsf.source_path` always
+uses — which can index arrays, filter, and reshape:
+
+```yaml
+field_mappings:
+  "custom:evidence":  "jsonata: $toTable(alerts.evidences.fields[])"
+  "custom:alert_name": "jsonata: $parseJson(alerts[0].evidences[0].data).source_json.AlertDisplayName"
+  severity:            "jsonata: priority = 'HIGH' ? 4 : 2"
+```
+
+Two Riposte extensions exist alongside the standard JSONata functions:
+
+| Function | What it does |
+|----------|--------------|
+| `$parseJson(str)` | Opens a JSON-encoded string — the original log line most SIEMs bury inside a JSON string field. |
+| `$toTable(x [, cols])` | Turns an array of objects into rows for a `table` custom field (a single object becomes key/value rows). `cols` keeps only the named columns. |
+
+Mind JSONata's singleton semantics: a path that finds **one** element unwraps
+it from its array. End the path with `[]` (`alerts.evidences.fields[]`) so a
+one-alert investigation still produces table rows instead of key/value pairs.
+
+**Convention: reshape in the mapper, not in the script.** A command script
+should return what the API gives, plus at most the flattening the API forces
+on it. Building display structures in Python (pre-joined rows, markdown
+summaries) welds them to the integration version — as expressions in the
+mapper they are visible, testable against the rule's captured samples
+(editor → Test), and an operator can adapt them without a package release.
+Pre-1.11 integrations that ship pre-built fields keep working; don't extend
+the pattern.
+
+An expression that fails costs its own field and never the alert — silently,
+by design. The mapper editor's **Test** button is where those errors surface,
+named by field, evaluated against the rule's last captured samples.
+
+---
+
 ## Adding this catalog to Riposte
 
 In Riposte → **Integrations → Marketplace → Add source**:
@@ -152,9 +222,11 @@ Then **Sync**. Discovered integrations appear in the marketplace, ready to insta
 
 1. Create `integrations/<id>/manifest.yaml` (one directory per integration).
 2. Add `scripts/<command-id>.py` only for script-based commands.
-3. Bump `version` (semver) on every change — Riposte tracks versions per source.
-4. Validate the YAML parses and `id`/`name` are set.
-5. Open a merge request.
+3. Add `mappers/<command-id>.yaml` for every ingestion command — reshape with
+   `jsonata:` expressions there rather than in the script (see above).
+4. Bump `version` (semver) on every change — Riposte tracks versions per source.
+5. Validate the YAML parses and `id`/`name` are set.
+6. Open a merge request.
 
 See [`templates/manifest.example.yaml`](templates/manifest.example.yaml) for a
 fully-commented starting point (that file is intentionally **not** ingested).