[ci] Re-enable pull_request on main.yml and emscripten.yml.

The cache-hygiene work on this branch needs real CI to validate
the cache-key cleanup, the source-`lib/` trim, the new
`clangInterpreter` ninja target, and the Build_LLVM_WASM
substitution-paren fix. main.yml exercises the native cache rows;
emscripten.yml exercises the WASM cache. Both were parked by the
parent ci-root-integration commit so the ROOT integration job
could iterate without contention -- partially restore that here,
scoped to the two workflows we need.

The other parked triggers (clang-format, clang-tidy-review,
markdown-linter) stay commented out: they don't exercise any of
this branch's changes, and keeping them off frees runner capacity
for the matrix. ci-root-integration should restore those when it
itself lands.

Author: vgvassilev

.github/workflows/emscripten.yml

@@ -1,10 +1,7 @@
 name: WASM
 on:
-  # XXX(ci-root-integration): pull_request disabled on this branch so the
-  # ROOT integration workflow gets the runner pool to itself while we
-  # iterate. Restore before landing.
-  # pull_request:
-  #   branches: [main]
+  pull_request:
+    branches: [main]
   push:
     branches: [main]
   release:

.github/workflows/main.yml

@@ -1,11 +1,8 @@
 name: CI
 
 on:
-  # XXX(ci-root-integration): pull_request disabled on this branch so the
-  # ROOT integration workflow gets the runner pool to itself while we
-  # iterate. Restore before landing.
-  # pull_request:
-  #   branches: [main]
+  pull_request:
+    branches: [main]
   push:
     branches: [main]
   release: