Feat v0.2.9 builtin extension retirement

Stop auto-seeding 6 chat-centric extensions on startup. Move them to
standard packages with js/ layout and "requires": ["chat"] metadata.
Delete SeedBuiltinPackages, seed tests, Dockerfile COPY, and
extensions/builtin/ directory.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/regex-tester/manifest.json

@@ -0,0 +1,39 @@
+{
+    "id": "regex-tester",
+    "name": "Regex Tester",
+    "version": "1.0.0",
+    "type": "extension",
+    "tier": "browser",
+    "author": "switchboard",
+    "description": "Tests regular expressions against input strings. Allows the LLM to verify regex patterns and see matches, groups, and indices.",
+    "requires": ["chat"],
+    "permissions": [],
+    "tools": [
+        {
+            "name": "regex_test",
+            "display_name": "Regex Tester",
+            "description": "Test a regular expression against one or more input strings. Returns match details including captured groups, indices, and whether each string matched. Use this to verify regex patterns instead of guessing.",
+            "parameters": {
+                "type": "object",
+                "properties": {
+                    "pattern": {
+                        "type": "string",
+                        "description": "The regular expression pattern (without delimiters), e.g. '^\\d{3}-\\d{4}$'"
+                    },
+                    "flags": {
+                        "type": "string",
+                        "description": "Regex flags, e.g. 'gi' for global+case-insensitive. Default: '' (no flags)"
+                    },
+                    "inputs": {
+                        "type": "array",
+                        "items": { "type": "string" },
+                        "description": "Array of test strings to match against the pattern"
+                    }
+                },
+                "required": ["pattern", "inputs"]
+            }
+        }
+    ],
+    "surfaces": [],
+    "settings": {}
+}