fix(import-markdown): CI測試登記 + .md目錄skip保護

- cli.ts: 讀檔前先stat確認isFile()，非檔案skip並log warn
- ci-test-manifest.mjs: 加入import-markdown.test.mjs到cli-smoke群組
- import-markdown.test.mjs: 新增測試『skip .md directory』

Ref: PR #482 issuecomment-4229349438 (app3apps)

Author: jlin53882

scripts/verify-ci-test-manifest.mjs

@@ -21,6 +21,7 @@ const EXPECTED_BASELINE = [
   { group: "core-regression", runner: "node", file: "test/strip-envelope-metadata.test.mjs", args: ["--test"] },
   { group: "cli-smoke", runner: "node", file: "test/cli-smoke.mjs" },
   { group: "cli-smoke", runner: "node", file: "test/functional-e2e.mjs" },
+  { group: "cli-smoke", runner: "node", file: "test/import-markdown/import-markdown.test.mjs", args: ["--test"] },
   { group: "core-regression", runner: "node", file: "test/retriever-rerank-regression.mjs" },
   { group: "core-regression", runner: "node", file: "test/smart-memory-lifecycle.mjs" },
   { group: "core-regression", runner: "node", file: "test/smart-extractor-branches.mjs" },