Add context packs impact analysis and message validation

specs/versions/0.1/message.md

@@ -0,0 +1,41 @@
+# GSP Message 0.1
+
+GSP Message is a lightweight agent communication format.
+
+It references GSP ids and does not replace `.gsp` files.
+
+```yaml
+gspMessageVersion: 0.1
+id: msg.implement.lottery_page
+from: planner
+to: implementer
+intent: implement
+entry: page.lottery.main
+stage: implement
+requires:
+  - ui.button.reward_primary
+  - feedback.positive
+contextPack:
+  mode: implement
+  depth: -1
+  budget: 12000
+```
+
+Required fields:
+
+| Field | Meaning |
+|---|---|
+| `gspMessageVersion` | Message protocol version. |
+| `id` | Message id. |
+| `from` | Sender agent or module id. |
+| `to` | Receiver agent or module id. |
+| `intent` | Task intent. |
+| `entry` | Entry GSP id. |
+
+Validation:
+
+```bash
+gsp message validate message.gspmsg --root .
+```
+
+The validator checks message version, required fields, intent, stage, entry GSP, and required GSP references.