ci: multi-toolchain coverage on Linux, remove continue-on-error tests

Linux CI now tests all 3 toolchains with freshly-built mcpp:
  - GCC 16.1.0: mcpp new → build → run
  - musl-gcc 15.1.0: mcpp new → build → run
  - LLVM 20.1.7: install + mcpp new → build → run

Remove continue-on-error "Fresh user experience" tests from all
3 CI workflows — moved to separate ci-fresh-install.yml (PR #63).
Those tests validate the xlings-distributed mcpp binary, not the
PR's code, so they don't belong in the main CI gate.

Author: Sunrisepeak

.github/workflows/ci-macos.yml

@@ -316,13 +316,4 @@ jobs:
           "$MCPP" --version
           echo ":: Self-host smoke PASS"
 
-      - name: Fresh user experience (xlings install mcpp → new → run)
-        continue-on-error: true
-        run: |
-          # Test real user flow with xlings-distributed mcpp.
-          # May fail if xlings mcpp version lacks recent fixes.
-          TMP=$(mktemp -d)
-          cd "$TMP"
-          "$MCPP" new hello
-          cd hello
-          "$MCPP" run
+      # Fresh user experience test moved to ci-fresh-install.yml (PR #63)

.github/workflows/ci-windows.yml

@@ -108,18 +108,7 @@ jobs:
           "$MCPP_SELF" --version
           echo ":: Self-host smoke PASS"
 
-      - name: Fresh user experience (xlings install mcpp → new → run)
-        continue-on-error: true
-        shell: bash
-        run: |
-          # Test the real user flow with the xlings-distributed mcpp.
-          # Currently xlings ships 0.0.17 which lacks Windows fixes.
-          # This step will auto-pass once xlings updates to 0.0.19+.
-          TMP=$(mktemp -d)
-          cd "$TMP"
-          "$MCPP" new hello
-          cd hello
-          "$MCPP" run
+      # Fresh user experience test moved to ci-fresh-install.yml (PR #63)
 
       - name: Package Windows release zip
         id: package