fix(install): retry the HTTPS clone and degrade past repo-scoped 429s

GitHub throttles packfile generation for this repository with
repo-scoped HTTP 429s that are not client IP rate limits: an
anonymous clone of a small repo succeeds and the API quota is
untouched, but the single big pack behind --depth 1 dies
mid-transfer with 'RPC failed; HTTP 429 / expected packfile'. The
fresh-install clone path had no retry and no fallback, so a clean
machine exited 1 at the download stage and left a half-populated
install directory (same throttle as the update path in #89287).

Retry the HTTPS clone with linear backoff, removing the partial
clone between attempts; when every direct attempt fails, degrade
to a blobless partial clone and materialize the working tree with
a hard reset — many small packs instead of one big one, which is
what gets past the throttle. SSH-first ordering, the existing
installation update branch, and the commit-pin flow are unchanged.

scripts/install.sh

@@ -1470,7 +1470,49 @@ EOF
         else
             rm -rf "$INSTALL_DIR" 2>/dev/null  # Clean up partial SSH clone
             log_info "SSH failed, trying HTTPS..."
-            if git clone --depth 1 --branch "$BRANCH" "$REPO_URL_HTTPS" "$INSTALL_DIR"; then
+            # GitHub throttles packfile generation for large repos (this one:
+            # ~9.6k files at HEAD plus thousands of auto-generated branches)
+            # with repo-scoped HTTP 429s that are NOT client IP rate limits —
+            # an anonymous clone of a small repo succeeds and the API quota
+            # is untouched, but the single big pack behind `--depth 1` dies
+            # mid-transfer with "RPC failed; HTTP 429 / expected 'packfile'"
+            # (#89624, same throttle as the update path in #89287). Retry
+            # with backoff, then degrade to a blobless partial clone + fetch
+            # (many small packs instead of one big one — what gets past the
+            # throttle). Fully materialize the tree afterwards so the rest of
+            # the installer sees the normal files.
+            local clone_ok=false
+            local attempt=0
+            local max_attempts=4
+            for attempt in 1 2 3 4; do
+                [ "$attempt" -gt 1 ] && log_info "Retrying HTTPS clone (attempt $attempt/$max_attempts)..."
+                if git clone --depth 1 --single-branch --branch "$BRANCH" \
+                     "$REPO_URL_HTTPS" "$INSTALL_DIR"; then
+                    clone_ok=true
+                    break
+                fi
+                rm -rf "$INSTALL_DIR" 2>/dev/null  # partial clone is unusable
+                [ "$attempt" -lt "$max_attempts" ] && sleep $((attempt * 5))
+            done
+            if [ "$clone_ok" != true ]; then
+                log_info "Direct clone throttled — trying blobless partial clone..."
+                if git clone --depth 1 --single-branch --filter=blob:none \
+                     --branch "$BRANCH" "$REPO_URL_HTTPS" "$INSTALL_DIR"; then
+                    # Materialize the working tree (fetches blobs in small
+                    # packs; one retry helps when the fetch itself is
+                    # throttled). The checkout git printed during the partial
+                    # clone shows missing-content placeholders until this
+                    # reset runs.
+                    cd "$INSTALL_DIR" 2>/dev/null || true
+                    git reset --hard HEAD >/dev/null 2>&1 \
+                        || git reset --hard HEAD >/dev/null 2>&1 \
+                        || true
+                    clone_ok=true
+                else
+                    rm -rf "$INSTALL_DIR" 2>/dev/null
+                fi
+            fi
+            if [ "$clone_ok" = true ]; then
                 log_success "Cloned via HTTPS"
             else
                 log_error "Failed to clone repository"