docs: add YAML capsule authoring MVP specification

docs/capsule-authoring-mvp.md

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+# YAML Capsule Authoring MVP
+
+## Overview
+Capsules are the fundamental unit of configuration in Syntra. This MVP defines the YAML schema for capsule authoring.
+
+## Capsule Schema v0.2
+
+```yaml
+# Required fields
+apiVersion: syntra/v0.2
+kind: Capsule
+metadata:
+  name: my-capsule
+  version: 1.0.0
+  description: "A brief description"
+  author: "your-name"
+  tags:
+    - utility
+    - automation
+
+# Capsule configuration
+spec:
+  # Input parameters
+  inputs:
+    - name: target_url
+      type: string
+      required: true
+      description: "URL to process"
+    - name: max_retries
+      type: integer
+      default: 3
+      description: "Maximum retry attempts"
+
+  # Execution steps
+  steps:
+    - name: fetch
+      action: http.get
+      params:
+        url: "{{inputs.target_url}}"
+        timeout: 30s
+
+    - name: transform
+      action: data.extract
+      params:
+        source: "{{steps.fetch.output}}"
+        format: json
+        path: ".data.items[]"
+
+    - name: output
+      action: data.format
+      params:
+        format: markdown
+        template: "results.md.j2"
+
+  # Output specification
+  outputs:
+    - name: result
+      type: string
+      from: "{{steps.output.result}}"
+
+  # Error handling
+  onError:
+    retry:
+      max: "{{inputs.max_retries}}"
+      delay: 5s
+    fallback:
+      action: notify
+      params:
+        channel: "#alerts"
+        message: "Capsule {{metadata.name}} failed"
+```
+
+## Validation Rules
+1. `apiVersion` must match supported versions
+2. `metadata.name` must be lowercase, hyphens only
+3. All required inputs must have values
+4. Steps must reference valid actions
+5. Output references must resolve
+
+## MVP Scope
+- [x] YAML schema definition
+- [x] Input/output specification
+- [ ] Runtime validation
+- [ ] Action library
+- [ ] Step execution engine