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

apps/api/src/models/tool_type.py

@@ -31,7 +31,6 @@ class ToolType(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     )
     readiness_probe: Mapped[dict | None] = mapped_column(JSON, nullable=True)
     required_variables: Mapped[list[str]] = mapped_column(JSON, default=list, nullable=False)
-    is_builtin: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
     created_by_id: Mapped[uuid.UUID | None] = mapped_column(
         UUID(),
         ForeignKey("users.id"),