ci(lotus): install pnpm directly — corepack shim breaks the matrix-js-sdk prepare

Run #1854 failed in `pnpm install --frozen-lockfile`: matrix-js-sdk is a
git dependency (upstream pins matrix-org/matrix-js-sdk#develop) that pnpm
must build from source, and that source's devEngines pins pnpm 11.9.0.
Under `corepack enable` the nested `pnpm install` runs the 11.21.0 shim,
which refuses to switch versions ("pnpm does not switch versions when
running under corepack") and the prepare step aborts. Reproduced locally
with a fresh store; with a real pnpm binary the nested install downloads
11.9.0 itself and succeeds.

Both jobs now `npm install -g pnpm@<version>` with the version read from
package.json's packageManager field.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

.gitea/workflows/ci.yml

@@ -48,8 +48,15 @@ jobs:
           registry-url: "https://code.lotusguild.org/api/packages/LotusGuild/npm/"
           scope: "@lotusguild"
 
-      - name: Enable corepack (pnpm from packageManager field)
-        run: corepack enable
+      # pnpm is installed directly (NOT via `corepack enable`): matrix-js-sdk
+      # is a git dependency that pnpm has to build from source, and its own
+      # devEngines pins a different pnpm (11.9.0). A corepack-shimmed pnpm
+      # refuses to switch versions for that nested install and the whole
+      # `pnpm install` fails (run #1854); a real pnpm binary downloads the
+      # pinned version itself. Version is read from package.json's
+      # packageManager field so it cannot drift from the lockfile.
+      - name: Install pnpm (version from packageManager field)
+        run: npm install -g "pnpm@$(node -p "require('./package.json').packageManager.split('@')[1].split('+')[0]")"
 
       - name: Install dependencies
         run: pnpm install --frozen-lockfile
@@ -110,8 +117,15 @@ jobs:
           registry-url: "https://code.lotusguild.org/api/packages/LotusGuild/npm/"
           scope: "@lotusguild"
 
-      - name: Enable corepack
-        run: corepack enable
+      # pnpm is installed directly (NOT via `corepack enable`): matrix-js-sdk
+      # is a git dependency that pnpm has to build from source, and its own
+      # devEngines pins a different pnpm (11.9.0). A corepack-shimmed pnpm
+      # refuses to switch versions for that nested install and the whole
+      # `pnpm install` fails (run #1854); a real pnpm binary downloads the
+      # pinned version itself. Version is read from package.json's
+      # packageManager field so it cannot drift from the lockfile.
+      - name: Install pnpm (version from packageManager field)
+        run: npm install -g "pnpm@$(node -p "require('./package.json').packageManager.split('@')[1].split('+')[0]")"
 
       - name: Install dependencies
         run: pnpm install --frozen-lockfile