docs(schema): complete extensions coverage (#496)

Author: yhay81

schema/Makefile

@@ -46,7 +46,8 @@ validate-fixtures:
 		fixtures/valid-unit.json \
 		fixtures/minimal-unit.json \
 		fixtures/flagged-unit.json \
-		fixtures/duplicate-flag.json
+		fixtures/duplicate-flag.json \
+		fixtures/extensions-unit.json
 	uv run --locked --project python check-jsonschema \
 		--schemafile node_discovery.json \
 		fixtures/node_discovery_minimal.json \

schema/README.md

@@ -26,6 +26,7 @@ A single unit of shared agent knowledge. This is the core data type; other schem
 | `tier` | `"local"` \| `"private"` \| `"public"` | no | Storage tier. |
 | `created_by` | string | no | Identifier of the agent or user that created this unit. |
 | `superseded_by` | string (`ku_<32 hex>`) | no | ID of the replacing knowledge unit, if any. |
+| `extensions` | Extensions | no | Implementation-specific namespaced fields; not portable across implementations. |
 | `flags` | Flag[] | no | Recorded flags against this unit. |
 
 #### Defined types
@@ -46,6 +47,8 @@ A single unit of shared agent knowledge. This is the core data type; other schem
 | `frameworks` | string[] | no | Frameworks. |
 | `pattern` | string | no | Reusable cross-cutting pattern. |
 
+**Extensions** — implementation-specific fields carried under namespaced keys, not portable across implementations. See the `Extensions` definition in `knowledge_unit.json` for the normative producer and consumer rules.
+
 **Evidence** — confidence and confirmation metrics.
 
 | Field | Type | Required | Description |
@@ -92,6 +95,7 @@ Request to propose a new knowledge unit.
 | `domains` | string[] (>= 1) | yes | At least one domain is required. |
 | `insight` | Insight | yes | Tripartite insight (defined in knowledge_unit.json). |
 | `context` | Context | no | Language, framework, and pattern context. |
+| `extensions` | Extensions | no | Implementation-specific namespaced fields (defined in knowledge_unit.json). |
 | `created_by` | string | no | Identifier of the proposing agent or user. |
 
 ---