feat: remove built-in tool types distinction

- Drop is_builtin column from tool_types table
- Remove built-in tool seeding from startup
- Remove is_builtin from API schemas and frontend types
- Update tool-types spec to reflect removal of built-in concept
- Add Alembic migration for column removal
- Update tests to work without built-in distinction

openspec/specs/tool-types/spec.md

@@ -2,7 +2,7 @@
 
 ### Requirement: Tool Type Model
 
-The system SHALL store tool type definitions in the database.
+The system SHALL store tool type definitions in the database without built-in vs custom distinction.
 
 #### Scenario: Create tool type
 - GIVEN an admin user
@@ -24,14 +24,8 @@ The system SHALL store tool type definitions in the database.
 
 ### Requirement: Built-in Tools
 
-The system SHALL include default tool types.
-
-#### Scenario: Built-in tools
-- GIVEN a fresh installation
-- THEN these tool types are pre-configured:
-  - code-server: VS Code in browser (port 8443, interfaces: ["web"])
-  - jupyter-notebook: Jupyter notebooks (port 8888, interfaces: ["web"])
-  - opencode: OpenCode agent environment (port 3000, interfaces: ["terminal", "web"])
+**Reason**: Built-in tools are now regular preconfigured tool types in the database, not special privileged types.
+**Migration**: Built-in tool types (code-server, jupyter-notebook, opencode) are seeded as regular database records during migration. They can be edited or deleted like any other tool type.
 
 ### Requirement: Template Validation