feat(extensions): add core selftest utility and migrate away from manual tests sandbox

Author: dhilipkumars

extensions/catalog.json

@@ -2,5 +2,18 @@
   "schema_version": "1.0",
   "updated_at": "2026-02-03T00:00:00Z",
   "catalog_url": "https://raw.githubusercontent.com/github/spec-kit/main/extensions/catalog.json",
-  "extensions": {}
-}
+  "extensions": {
+    "selftest": {
+      "name": "Spec Kit Self-Test Utility",
+      "version": "1.0.0",
+      "description": "Verifies catalog extensions by programmatically walking through the discovery, installation, and registration lifecycle.",
+      "author": "spec-kit-core",
+      "repository": "https://github.com/github/spec-kit",
+      "tags": [
+        "testing",
+        "core",
+        "utility"
+      ]
+    }
+  }
+}

extensions/selftest/commands/selftest.md

@@ -0,0 +1,62 @@
+---
+description: "Validate the lifecycle of an extension from the catalog."
+---
+
+# Extension Self-Test: `$ARGUMENTS`
+
+This command drives a self-test simulating the developer experience with the `$ARGUMENTS` extension.
+
+## Goal
+
+Validate the end-to-end lifecycle (discovery, installation, registration) for the extension: `$ARGUMENTS`.
+If `$ARGUMENTS` is empty, you must tell the user to provide an extension name, for example: `/speckit.selftest linear`.
+
+## Steps
+
+### Step 1: Catalog Discovery Validation
+
+Check if the extension exists in the Spec Kit catalog.
+Execute this command and verify that `$ARGUMENTS` appears in the results. If the command fails or returns no results, fail the test.
+
+```bash
+specify extension search $ARGUMENTS
+```
+
+### Step 2: Simulate Installation
+
+Simulate adding the extension to the current workspace configuration.
+
+```bash
+specify extension add $ARGUMENTS
+```
+
+### Step 3: Registration Verification
+
+Once the `add` command completes, verify the installation by checking the project configuration.
+Use terminal tools (like `cat`) to verify that the following file contains an enabled record for `$ARGUMENTS`.
+
+```bash
+cat .specify/extensions.yml
+```
+
+### Step 4: Verification Report
+
+Analyze the standard output of the three steps. 
+Generate a terminal-style test output format detailing the results of discovery, installation, and registration. Return this directly to the user.
+
+Example output format:
+```text
+============================= test session starts ==============================
+collected 3 items
+
+test_selftest_discovery.py::test_catalog_search [PASS/FAIL]
+  Details: [Provide execution result of specify extension search]
+
+test_selftest_installation.py::test_extension_add [PASS/FAIL]
+  Details: [Provide execution result of specify extension add]
+
+test_selftest_registration.py::test_config_verification [PASS/FAIL]
+  Details: [Provide execution result of reading .specify/extensions.yml]
+
+============================== [X] passed in ... ==============================
+```

extensions/selftest/extension.yml

@@ -0,0 +1,16 @@
+schema_version: "1.0"
+extension:
+  id: selftest
+  name: Spec Kit Self-Test Utility
+  version: 1.0.0
+  description: Verifies catalog extensions by programmatically walking through the discovery, installation, and registration lifecycle.
+  author: spec-kit-core
+  repository: https://github.com/github/spec-kit
+  license: MIT
+requires:
+  speckit_version: ">=0.1.0"
+provides:
+  commands:
+    - name: speckit.selftest
+      file: commands/selftest.md
+      description: Validate the lifecycle of an extension from the catalog.