Add collection migration example and guide

examples/go-library/prepare/collections/review/collection.yaml

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+backends:
+  offline:
+    endpoint: https://review.example.invalid/v1
+profiles:
+  audit:
+    backend: review/offline
+    model: review-model
+prompts:
+  check:
+    default_profile: review/audit
+    description: Inspect a review item without contacting a model provider.
+    messages:
+      - role: user
+        content: Review the supplied synthetic example.
+    output:
+      format: text
+      validation_mode: none

examples/go-library/prepare/collections/support/collection.yaml

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+version: "1"
+configkit:
+  imports:
+    - path: fragments/resources.yaml
+release_version: "1.4.0"

examples/go-library/prepare/collections/support/fragments/resources.yaml

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+backends:
+  offline:
+    endpoint: https://example.invalid/v1
+profiles:
+  base:
+    backend: support/offline
+    model: offline-model
+    timeout_seconds: 90
+  fast:
+    extends: base
+    timeout_seconds: 60
+prompts:
+  prepare:
+    version: "1.0.0"
+    default_profile: support/fast
+    description: Prepare a prompt without contacting a model provider.
+    inputs:
+      - name: note
+        required: true
+        content_type: text/plain
+    messages:
+      - role: system
+        content: Summarize the note in one sentence.
+      - role: user
+        content_file: ../messages/note.md
+    output:
+      format: text
+      validation_mode: basic

examples/go-library/prepare/collections/support/messages/note.md

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+{{input "note"}}

examples/go-library/prepare/collections/support/operator-overrides.yaml

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+# A later input intentionally replaces only the inherited profile timeout.
+profiles:
+  fast:
+    timeout_seconds: 30