Address Copilot review feedback

- Fix conda channel name: 'default' -> 'defaults'
- Add permissions block to ci.yml (actions: read for artifact download)
- Tighten publish.yml permissions: contents: write -> contents: read

Author: mmcky

.github/workflows/ci.yml

@@ -1,6 +1,10 @@
 name: Build Project [using jupyter-book]
 on: [pull_request]
 
+permissions:
+  contents: read
+  actions: read
+
 jobs:
   preview:
     runs-on: ubuntu-latest

.github/workflows/publish.yml

@@ -5,7 +5,7 @@ on:
       - 'publish*'
 
 permissions:
-  contents: write
+  contents: read
   actions: read
   pages: write
   id-token: write

environment.yml

@@ -1,6 +1,6 @@
 name: quantecon
 channels:
-  - default
+  - defaults
 dependencies:
   - python=3.13
   - anaconda=2025.12