Mirror: source from drydock, use filter-repo, materialize mode/

- Default source ref mainmast -> drydock (mainmast held for team review).
- Extract via `git filter-repo` (~1s) instead of `git subtree split`, which
  re-walks all ~12k monorepo commits and takes ~an hour for the same result.
- After extraction, copy the shared codemirror-mode sibling in as a real
  `mode/` dir (force-added past .gitignore) so the mirror is a self-contained,
  Heroku-deployable slice -- matching the horizon deploy's slice fix.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: jpolitz

.github/workflows/monorepo-mirror.yml

@@ -1,24 +1,32 @@
 name: Mirror monorepo code.pyret.org subtree
 
-# Generates a flat, deployable mirror of the code.pyret.org/ subfolder of the
-# brownplt/pyret-lang monorepo, with history preserved, on the
-# `monorepo-mirror` branch of THIS repo.
+# Generates a deployable, history-preserving mirror of the code.pyret.org/
+# subfolder of the brownplt/pyret-lang monorepo on the `monorepo-mirror` branch
+# of THIS repo, so downstream forks (e.g. Bootstrap/PBO) can keep their
+# pull -> push -> Heroku "Deploy" workflow once CPO lives in the monorepo.
 #
-# Runs in brownplt/code.pyret.org so it can push to a branch here using the
+# Runs in brownplt/code.pyret.org so it can push to a branch here with the
 # built-in GITHUB_TOKEN -- no PAT or deploy key needed. The monorepo is public,
-# so it is checked out read-only with that same token.
+# so it's checked out read-only with that same token.
 #
-# Manual-only for now: trigger from the Actions tab and pick the source ref.
-# Uncomment the `schedule` block below to run it automatically once this is
-# trusted.
+# Extraction uses `git filter-repo` (sub-second). `git subtree split` produces
+# the same result but re-walks all ~12k monorepo commits and takes ~an hour, so
+# it's not viable here. The mirror is the history of code.pyret.org/ rewritten to
+# the repo root, plus one commit that copies the shared ../codemirror-mode
+# sibling in as a real `mode/` directory: CPO's Makefile reads the CodeMirror
+# mode from a local `mode/` (a gitignored symlink in the monorepo), so the slice
+# must ship it as a real dir for `make web` to build standalone on Heroku. (The
+# brand images the build needs are already vendored into src/web/img.)
+#
+# Manual-only for now; uncomment `schedule` to run automatically once trusted.
 
 on:
   workflow_dispatch:
     inputs:
       source_ref:
         description: "Branch/ref of brownplt/pyret-lang to mirror"
         required: true
-        default: mainmast
+        default: drydock
   # schedule:
   #   - cron: "0 7 * * *"   # daily at 07:00 UTC
 
@@ -37,16 +45,34 @@ jobs:
         uses: actions/checkout@v4
         with:
           repository: brownplt/pyret-lang
-          ref: ${{ github.event.inputs.source_ref || 'mainmast' }}
+          ref: ${{ github.event.inputs.source_ref || 'drydock' }}
           fetch-depth: 0
           path: monorepo
 
-      - name: Split code.pyret.org subtree (preserves history)
+      - name: Install git-filter-repo
+        run: |
+          sudo apt-get update -qq
+          sudo apt-get install -y git-filter-repo
+          git filter-repo --version
+
+      - name: Extract code.pyret.org (history-preserving) + materialize mode/
         working-directory: monorepo
         run: |
-          git subtree split --prefix=code.pyret.org -b mirror-split
-          echo "Split tip: $(git rev-parse mirror-split)"
-          git log --oneline -5 mirror-split
+          set -euo pipefail
+          # Stash the shared codemirror-mode sibling before filter-repo prunes it.
+          cp -r codemirror-mode /tmp/codemirror-mode
+          # Rewrite history so code.pyret.org/ becomes the repo root.
+          git filter-repo --subdirectory-filter code.pyret.org --force
+          # CPO reads the CodeMirror mode from a local `mode/` (a gitignored
+          # symlink in the monorepo). Ship a real copy so a downstream Heroku
+          # build of this mirror is self-contained.
+          rm -rf mode
+          cp -r /tmp/codemirror-mode mode
+          git config user.name "pyret-mirror bot"
+          git config user.email "119891+jpolitz@users.noreply.github.com"
+          git add -f mode
+          git commit -m "Materialize codemirror-mode for standalone deploy"
+          git log --oneline -5
 
       - name: Push to monorepo-mirror branch of this repo
         working-directory: monorepo
@@ -55,5 +81,5 @@ jobs:
         run: |
           git push --force \
             "https://x-access-token:${GH_TOKEN}@github.com/${{ github.repository }}.git" \
-            mirror-split:monorepo-mirror
-          echo "Pushed mirror-split -> monorepo-mirror"
+            HEAD:monorepo-mirror
+          echo "Pushed HEAD -> monorepo-mirror"