fix: add scoped BuildKit GC entry

docs/reference/gc.md

@@ -138,6 +138,7 @@ Worktree 清理由 `--include-merged-worktrees` 显式启用，只扫描 `config
 - systemd journal vacuum 到目标上限。
 - Docker `json-file` 日志截断。
 - Docker BuildKit cache prune 只清理 owning YAML `hostDockerGc.buildCachePrune.until` 声明年龄之外的 cache；显式 `--build-cache-until` 可覆盖本次计划，不清理 images、containers 或 volumes。
+- `--build-cache-only` 提供短连接 scoped plan/run，跳过 PVC、registry 和全盘 collectors；确认态仍要求 `--confirm` 并复用同一 YAML 筛选器。
 - apt archive clean。
 - allowlisted `/tmp` 诊断目录删除。
 - 受限 core dump 删除。

scripts/src/gc-remote-runner.py

@@ -1071,6 +1071,25 @@ def parse_docker_build_cache(text):
         return {"sizeBytes": size, "reclaimableBytes": reclaim}
     return None
 
+def build_cache_only_payload(observed_at):
+    disk_before = df_snapshot()
+    section = host_docker_gc_section("buildCachePrune")
+    until = str(OPTIONS.get("buildCacheUntil") or section.get("until") or "24h")
+    if not host_docker_gc_enabled("buildCachePrune"):
+        return {"ok": False, "action": "gc remote %s" % ACTION, "providerId": PROVIDER_ID, "scope": "build-cache-only", "dryRun": ACTION == "plan", "mutation": False, "diagnosis": host_docker_gc_reason("buildCachePrune", "BuildKit cache GC is disabled by YAML")}
+    system_df = command(["docker", "system", "df"], 8)
+    if system_df["exitCode"] != 0:
+        return {"ok": False, "action": "gc remote %s" % ACTION, "providerId": PROVIDER_ID, "scope": "build-cache-only", "dryRun": ACTION == "plan", "mutation": False, "diagnosis": bounded(system_df)}
+    cache = parse_docker_build_cache(system_df["stdout"])
+    candidate = None
+    if cache is not None and cache["reclaimableBytes"] > 0:
+        candidate = {"id": "docker-builder:prune", "kind": "docker-build-cache-prune", "risk": "low", "sizeBytes": cache["sizeBytes"], "estimatedReclaimBytes": cache["reclaimableBytes"], "action": {"command": ["docker", "builder", "prune", "--all", "--force", "--filter", "until=%s" % until]}}
+    payload = {"ok": True, "action": "gc remote %s" % ACTION, "providerId": PROVIDER_ID, "scope": "build-cache-only", "configSource": "config/unidesk-cli.yaml#gc.remote.targets.%s.hostDockerGc.buildCachePrune" % PROVIDER_ID, "dryRun": ACTION == "plan", "mutation": ACTION == "run" and candidate is not None, "observedAt": observed_at, "diskBefore": disk_before, "until": until, "candidates": [candidate] if candidate else []}
+    if ACTION == "run" and candidate is not None:
+        payload["result"] = execute(candidate)
+        payload["diskAfter"] = df_snapshot()
+    return payload
+
 def docker_containers():
     ps = command(["docker", "ps", "-qa", "--no-trunc"], 5)
     if ps["exitCode"] != 0 or not ps["stdout"].strip():
@@ -2776,6 +2795,9 @@ def main():
         else:
             emit_json(start_memory_pressure_job(observed_at, visible), persist_large=True)
         return 0
+    if bool(OPTIONS.get("buildCacheOnly")) and ACTION in set(["plan", "run"]):
+        emit_json(build_cache_only_payload(observed_at), persist_large=False)
+        return 0
     if (bool(OPTIONS.get("registry")) or bool(OPTIONS.get("hwlabRegistry"))) and ACTION == "plan":
         emit_json(registry_plan_payload(observed_at), persist_large=True)
         return 0

scripts/src/gc-remote.ts

@@ -38,6 +38,7 @@ interface RemoteGcOptions {
   historyLimit: number;
   saveSnapshot: boolean;
   memoryPressureOnly: boolean;
+  buildCacheOnly: boolean;
 }
 
 const DEFAULT_REMOTE_OPTIONS: RemoteGcOptions = {
@@ -69,6 +70,7 @@ const DEFAULT_REMOTE_OPTIONS: RemoteGcOptions = {
   historyLimit: 12,
   saveSnapshot: true,
   memoryPressureOnly: false,
+  buildCacheOnly: false,
 };
 
 const GC_CONFIG_RELATIVE_PATH = "config/unidesk-cli.yaml";
@@ -298,6 +300,8 @@ function parseRemoteGcOptions(args: string[]): RemoteGcOptions {
       options.saveSnapshot = false;
     } else if (arg === "--memory-pressure-only") {
       options.memoryPressureOnly = true;
+    } else if (arg === "--build-cache-only") {
+      options.buildCacheOnly = true;
     } else {
       throw new Error(`unknown gc remote option: ${arg}`);
     }