feat: sdk subcommand to run local conformance against any SDK ref (#277)

* feat: add sdk subcommand to run conformance against any SDK ref (#250)

* Revise sdk runner: explicit --mode, KNOWN_SDKS-only config, v1/v2 entries

Addresses review feedback on the sdk subcommand:

- Require --mode (client|server) and remove "both". Each invocation now
  tests exactly one side with its own exit code; the old default ran
  client then server but combined exit codes with ||=, which skipped the
  server side entirely whenever the client run failed.
- Resolve build/run config from KNOWN_SDKS + CLI flags only; drop the
  conformance.config.yaml loader (no SDK ships one yet). The Zod schema
  stays as the type for the built-in entries.
- Split the typescript entry by major version: typescript-sdk (v2/main,
  pnpm install + build:all, expected-failures.yaml) and typescript-sdk-v1
  (v1.x, npm ci + build, conformance-baseline.yml). An entry may set
  `repo` (real clone target for an alias) and `defaultRef` (branch used
  when no @ref is given). parseSdkSpec now leaves ref undefined when
  omitted so defaultRef can apply; a trailing @ is treated as no ref.
- Key the clone cache by ref (<repo>/<ref>) so different refs of the same
  repo no longer share one checkout.
- Bound the server readiness probe with a per-request AbortSignal timeout
  so a server that accepts the socket but never responds can't hang past
  the overall deadline.

* fix(sdk-runner): resolve -o to absolute path; add --expected-failures override; replaceAll for safeName

vitest.config.ts

@@ -5,7 +5,7 @@ export default defineConfig({
     globals: true,
     environment: 'node',
     include: ['**/*.test.ts'],
-    exclude: ['**/node_modules/**', 'dist'],
+    exclude: ['**/node_modules/**', 'dist', '.sdk-under-test'],
     // Run test files sequentially to avoid port conflicts
     fileParallelism: false,
     // Increase timeout for server tests in CI