feat: add --expected-failures baseline and composite GitHub Action (#113)

Add support for SDKs that don't yet pass all conformance tests to still
run conformance in CI without failing, by specifying a YAML baseline file
of known/expected failures.

CLI changes:
- Add --expected-failures <path> flag to both server and client commands
- Exit 0 if only expected failures occur
- Exit 1 if unexpected failures occur (regression)
- Exit 1 if an expected failure now passes (stale baseline, must update)

GitHub Action (action.yml):
- Composite action that builds from source and runs conformance tests
- SDKs reference it as: uses: modelcontextprotocol/conformance@<ref>
- Inputs: mode, url, command, expected-failures, suite, scenario, etc.

New files:
- src/expected-failures.ts: YAML loading, baseline evaluation, reporting
- src/expected-failures.test.ts: 16 unit tests
- action.yml: composite GitHub Action

Closes #99

package-lock.json

@@ -14,6 +14,7 @@
         "eventsource-parser": "^3.0.6",
         "express": "^5.1.0",
         "jose": "^6.1.2",
+        "yaml": "^2.8.2",
         "zod": "^3.25.76"
       },
       "bin": {
@@ -5645,6 +5646,21 @@
       "integrity": "sha1-tSQ9jz7BqjXxNkYFvA0QNuMKtp8=",
       "license": "ISC"
     },
+    "node_modules/yaml": {
+      "version": "2.8.2",
+      "resolved": "https://registry.npmjs.org/yaml/-/yaml-2.8.2.tgz",
+      "integrity": "sha512-mplynKqc1C2hTVYxd0PU2xQAc22TI1vShAYGksCCfxbn/dFwnHTNi1bvYsBTkhdUNtGIf5xNOg938rrSSYvS9A==",
+      "license": "ISC",
+      "bin": {
+        "yaml": "bin.mjs"
+      },
+      "engines": {
+        "node": ">= 14.6"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/eemeli"
+      }
+    },
     "node_modules/yocto-queue": {
       "version": "0.1.0",
       "resolved": "https://registry.npmjs.org/yocto-queue/-/yocto-queue-0.1.0.tgz",