Standardize book-publish-live flag to site_only (was skip_version)

Aligns book-publish-live.yml with the rest of the publish workflows
(instructors, kits, labs, mlsysim, slides, staffml, tinytorch), which
all use:

  site_only:
    type: boolean
    default: false

The book workflow was the lone outlier carrying:

  skip_version:
    type: choice
    options: [no, yes]
    default: 'no'

Behavior is unchanged: site_only=true skips the DOI rewrite, tag
creation, release-notes generation, and GitHub Release creation while
still running the full HTML/PDF/EPUB build and gh-pages deploy.

Changes:
  - workflow_dispatch.inputs.skip_version → site_only (choice → boolean)
  - workflow_call.inputs.skip_version → site_only (string → boolean)
  - 5x `inputs.skip_version != 'yes'` → `inputs.site_only != 'true'`
  - Inline comment update describing the skip-cascade guard

Per the project's release convention, workflow file changes must be
on main before any dispatch that uses the new input name. Merge dev
into main and push both branches before the next workflow_dispatch
that wants to set site_only=true.

.github/workflows/book-publish-live.yml

@@ -184,14 +184,11 @@ on:
           - 'vol1'
           - 'vol2'
         default: 'all'
-      skip_version:
-        description: 'Skip the version bump? When yes, the merge to main, full build (HTML/PDF/EPUB), and gh-pages deploy all run normally, but the doi rewrite, tag creation, release-notes generation, and GitHub Release creation are skipped. Use this for post-release content tweaks where the version should stay at its current value.'
+      site_only:
+        description: 'Website-only deploy? When true, the merge to main, full build (HTML/PDF/EPUB), and gh-pages deploy all run normally, but the DOI rewrite, tag creation, release-notes generation, and GitHub Release creation are skipped. Use this for post-release content tweaks where the version should stay at its current value.'
         required: false
-        type: choice
-        options:
-          - 'no'
-          - 'yes'
-        default: 'no'
+        type: boolean
+        default: false
   workflow_call:
     inputs:
       description:
@@ -233,10 +230,10 @@ on:
         type: string
         required: false
         default: 'all'
-      skip_version:
-        type: string
+      site_only:
+        type: boolean
         required: false
-        default: 'no'
+        default: false
 
 permissions:
   contents: write
@@ -593,7 +590,7 @@ jobs:
           } >> "$GITHUB_OUTPUT"
 
       - name: 🛡️ No-downgrade guard (per-volume)
-        if: inputs.skip_version != 'yes'
+        if: inputs.site_only != 'true'
         env:
           # Step `run:` blocks don't inherit shell variables across steps;
           # surface the per-volume outputs as env so the guard can read them.
@@ -857,7 +854,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 10
     needs: [validate-inputs, pre-flight-checks]
-    if: inputs.confirm == 'PUBLISH' && inputs.testing_mode != 'yes' && inputs.skip_version != 'yes'
+    if: inputs.confirm == 'PUBLISH' && inputs.testing_mode != 'yes' && inputs.site_only != 'true'
 
     steps:
       - name: 📥 Checkout repository
@@ -1081,7 +1078,7 @@ jobs:
     timeout-minutes: 10
     needs: merge-to-main
     # always() guards against the skip cascade that happens when update-version
-    # is skipped (skip_version=yes) and merge-to-main runs under its own
+    # is skipped (site_only=true) and merge-to-main runs under its own
     # always() — without this, GitHub propagates the skip through the chain
     # and the build never runs, leaving the gh-pages deploy stale.
     if: |
@@ -1133,7 +1130,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 10
     needs: [validate-inputs, download-and-deploy-artifacts]
-    if: inputs.confirm == 'PUBLISH' && inputs.testing_mode != 'yes' && inputs.skip_version != 'yes'
+    if: inputs.confirm == 'PUBLISH' && inputs.testing_mode != 'yes' && inputs.site_only != 'true'
 
     steps:
       - name: 📥 Checkout repository
@@ -1713,7 +1710,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 30
     needs: [validate-inputs, download-and-deploy-artifacts]
-    if: inputs.confirm == 'PUBLISH' && inputs.testing_mode != 'yes' && inputs.skip_version != 'yes'
+    if: inputs.confirm == 'PUBLISH' && inputs.testing_mode != 'yes' && inputs.site_only != 'true'
 
     steps:
       - name: 📥 Checkout repository
@@ -1994,7 +1991,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 10
     needs: [validate-inputs, generate-release-notes, create-tag]
-    if: inputs.confirm == 'PUBLISH' && inputs.testing_mode != 'yes' && inputs.skip_version != 'yes'
+    if: inputs.confirm == 'PUBLISH' && inputs.testing_mode != 'yes' && inputs.site_only != 'true'
 
     steps:
       - name: 📥 Checkout repository